Re: [PATCH v1 05/17] xen/riscv: implement virtual APLIC MMIO emulation

[Oleksii Kurochko <oleksii.kurochko@xxxxxxxxx>]

On 8/11/26 5:29 PM, Baptiste Le Duc wrote:
> On 2026-08-11 16:36 +0200, Oleksii Kurochko wrote:
> > On 8/11/26 11:21 AM, Baptiste Le Duc wrote:
> > > On 2026-08-07 18:08:21+02:00, Oleksii Kurochko wrote:
> > > > On 8/6/26 4:28 PM, Jan Beulich wrote:
> > > >
> > > > > On 20.07.2026 18:02, Oleksii Kurochko wrote:
> > > > >
> > > > > For this tag to have any meaning, it should move ahead of the --- above;
> > > > > the explanations ...
> > > > >
> > > > >
> > > > > ... here rather explain the restriction on the R-b, not its odd placement.
> > > > >
> > > > >
> > > > > As this looks to be recurring - please get versioning of your series right.
> > > > > The series is supposedly v1, but here you give the impression of it being
> > > > > v3. If there really was an earlier v2 posting, why isn't the entire series
> > > > > here v3?
> > > > It is v3 before before it was a part of another patch series connected
> > > > to dom0less config enablement.
> > > >
> > > > Would it be better to just write in "Change in v3" that it is moved from
> > > > another patch series + link to that patch series? Or it will be enough
> > > > just to drop "Changes in v2 and v1" and just start from v1?
> > > >
> > > > > PLease can you, before submitting, self-review your patches? I'm really
> > > > > getting tired of having to repeatedly point out basic style issues, like
> > > > > the overlong line here.
> > > > Sorry for that, I will write an extra checker for such cases to not miss
> > > > them.
> > > >
> > > > > It extends to the other local variables here, but I'll use these two to
> > > > > try to make my point: I'm struggling to associate the names with the
> > > > > values they are set to. Likely "hxw" is an abbreviation of hart index
> > > > > width, but (a) what's the leading 'l' then and (b) why is there no 'g'
> > > > > in "hhxw"? By using hard to grasp names, you make it hard to actually
> > > > > understand the subsequent expressions, in particular ...
> > > > The names it taken directly from AIA spec:
> > > >
> > > > The use of this value and fields HHXS (High Hart Index Shift), LHXS (Low
> > > > Hart Index Shift), HHXW (High Hart Index Width), and LHXW (Low Hart
> > > > Index Width) for determining target addresses for MSIs is described
> > > > later, in Section 4.9.1.
> > > >
> > > > The AIA specification interprets the machine-level hart index as a
> > > > combination of the **group index** (`g`) and the **hart index within the
> > > > group** (`h`), according to the following formulas:
> > > >
> > > > ```
> > > > (1) g = (machine-level hart index >> LHXW) & (2^HHXW − 1)
> > > > (2) h = machine-level hart index & (2^LHXW − 1)
> > > > ```
> > > >
> > > > (In our case, the machine-level hart index is equal to `mhartid`, i.e.
> > > > the hart index.)
> > > Therefore, if I understand correclty, if we take the Hart Index as
> > > defined in the AIA spec, we should have:
> > > Hart Index = (g << LHXW) | h
> > > Is it correct?
> > Yes.
> >
> > But note that in the current version of aplic_hart_field(), hart_id is
> > passed directly, so there is no need to extract h as described in the
> > AIA specification. We only need to concatenate it with the group index
> > that we have already extracted.
> >
> > This is partly because aplic_hart_field() uses only .base_addr, which
> > does not contain hart_index.
> >
> > If we want to follow the AIA specification fully, using its terminology,
> > the code should look something like:
> >
> > static unsigned long aplic_hart_field(unsigned int cpu)
> > {
> >       const struct imsic_config *imsic = imsic_get_config();
> >       const struct imsic_msi *msi = &imsic->msi[cpu];
> Could you please specify how this function will be used and when? It's
> hard for me to understand how imsic->msi[cpu] is filled.
imsic->msi[] is filled during IMSIC initialization in imsic_init(), 
based on the MMIO regset specified in the IMSIC node’s reg property and 
the number of parents specified in the interrupts-extended property. 
This is explained to some extent in the comment above local target_addr 
in aplic_hart_field() (a little further down).
I am not 100% sure that I fully understand the connection between your 
question and the sentence after it, but I planned to write the following 
above the function declaration:

/*
 * The arrangement of IMSIC interrupt files in MMIO space follows a 
topology
 * defined by the RISC-V AIA specification. An IMSIC group is a set of
 * interrupt files (e.g., in a cluster or socket) co-located in memory.
 *
 * The physical address of an outgoing MSI is calculated by bitwise ORing a
 * Base Physical Page Number (Base PPN) with the Group Index (g), the Hart
 * Index (h) and, for a supervisor-level interrupt domain, the Guest Index:
 *
 *   ( Base PPN | (g << (HHXS + 12)) | (h << LHXS) | guest ) << 12
 *
 * where Base PPN, HHXS, LHXS, HHXW and LHXW come from the 
{m,s}msiaddrcfg[h]
 * registers of the interrupt domain that sends the MSI:
 *
 * XLEN-1           HHXS+24             LHXS+12          12          0
 * |                |                   |                |           |
 * -------------------------------------------------------------------
 * |xxxx|Group Index|xxxxxxxx|Hart Index|xxxx|Guest Index|     0     |
 * -------------------------------------------------------------------
 *
 * - xxxx: the remaining bits of the Base PPN. The specification 
requires the
 *   Base PPN to have zeros in the positions where the indices are OR-ed.
 * - Group Index (g): placed at bit (HHXS + 24) of the physical address.
 * - Hart Index (h): placed at bit (LHXS + 12) of the physical address.
 * - Guest Index: selects one of the 4 KiB pages right above the hart's own
 *   supervisor-level file, i.e. it starts at bit 12; LHXS must 
therefore be
 *   at least as large as the number of guest index bits.
 * - Bits 11:0: always zero because IMSIC files are 4 KiB page-aligned.
 *
 * For wired interrupts in MSI delivery mode (domaincfg.DM = 1) the APLIC
 * builds that address itself from the "Hart Index" field (bits 31:18) 
of the
 * corresponding target[i] register. That field holds a hart index 
*number*,
 * in which both indices are packed adjacently:
 *
 * 13          lhxw+hhxw   lhxw       0
 * |           |           |          |
 * ------------------------------------
 * |     0     |Group Index|Hart Index|
 * ------------------------------------
 *
 * - lhxw (Low Hart Index Width): the number of bits used for the hart 
number
 *   within a group.
 * - hhxw (High Hart Index Width): the number of bits used for the group
 *   number; the remaining bits of the field must be zero.
 *
 * The Guest Index isn't a part of it: for a supervisor-level interrupt 
domain
 * it has its own field (bits 17:12) in target[i].
 *
 * Because there are "xxxx" gaps (Base PPN bits) between the indices in the
 * physical address (depending on HHXS and LHXS), software must extract the
 * group and hart components separately and pack them into the 
APLIC-defined
 * Hart Index format to ensure correct MSI targeting.
 */

Does it answer your question?

> >       unsigned int lhxs = imsic->guest_index_bits;
> >       unsigned int lhxw = imsic->hart_index_bits;
> >       unsigned int hhxw = imsic->group_index_bits;
> >       unsigned int hhxs =
> >           imsic->group_index_shift - APLIC_xMSICFGADDR_PPN_SHIFT * 2;
> >       /*
> >        * msi->base_addr is the base of the MMIO regset this CPU's interrupt
> >        * files live in, and one regset can cover several harts; msi->offset
> >        * selects this CPU's block inside it.  The hart index bits are part of
> >        * that offset, so both indexes have to be derived from the full
> > address.
> >        */
> >       paddr_t target_addr = msi->base_addr + msi->offset;
> >       unsigned long tppn = target_addr >> APLIC_xMSICFGADDR_PPN_SHIFT;
> >       unsigned long group_index =
> >           (tppn >> APLIC_xMSICFGADDR_PPN_HHX_SHIFT(hhxs)) &
> >           APLIC_xMSICFGADDR_PPN_HHX_MASK(hhxw);
> >       unsigned long hart_index =
> >           (tppn >> APLIC_xMSICFGADDR_PPN_LHX_SHIFT(lhxs)) &
> >           APLIC_xMSICFGADDR_PPN_LHX_MASK(lhxw);
> >
> >       return (group_index << lhxw) | hart_index;
> > }
> >
> > (note that during writing that I found an issue, it should be really
> > passed Xen cpu id, not hartid as msi[] is iterated through Xen cpu id so
> > I've taken that into account when wrote an implementation mentioned above)
> >
> > Generally I think I am okay with both version of how to get hart_index
> > (or pass it by an argument or extract it).
> >
> > > > For systems that use IMSIC groups, the IMSIC address layout is defined
> > > > by the following parameters:
> > > >
> > > > * `lhxw` (Low Hart Index Width, or *k*): the number of bits used for the
> > > > hart number within a group.
> > > > * `hhxw` (High Hart Index Width, or *j*): the number of bits used for
> > > > the group number.
> > > Is group number appelation equivalent to group index?
> > >
> > > I think with if what I wrote above is correct, the proper definition for
> > > `hhxw` and `hhxs` should be:
> > > * `hhxw` (High Hart Index Width, or *j*): the number of bits used for
> > > the `Hart Index` field within the physical address.
> > > > * `hhxs` (High Hart Index Shift): the bit offset of the combined
> > > > hart/group index field within the physical address.
> > > * `hhxs` (High Hart Index Shift): the bit offset of the `Hart Index`
> > > field within the physical address.
> > > > To extract the group index, we first shift the address by `hhxs` so that
> > > > the group index bits are aligned, and then apply a mask derived from
> > > > `hhxw` to isolate those bits.
> > > >
> > > > The hardware performs the same operation to extract the hart index from
> > > > the MSI address. However, in our case we already know which hart should
> > > > receive the interrupt (`hartid`), so there is no need to extract the
> > > > hart index from the base address. We only need to recover the group
> > > > index and combine it with `hartid` to construct the value expected by
> > > > the `target` register.
> > > Why don't we direclty extract the Hart Index as target directly needs it
> > > as explained in the 4.5.16.2 point of the AIA spec:
> > > target[31:18] = Hart Index
> > > target[17:12] = Guest Index
> > > target[10:0] = EEID
> > > It'd be easier as we just have to do shift from HHXS and apply HHXW.
> >   From IMSIC's DT-binding description we have:
> >
> >     XLEN-1            > (HART Index MSB)                  12    0
> >     |                  |                                  |     |
> >     -------------------------------------------------------------
> >     |xxxxxx|Group Index|xxxxxxxxxxx|HART Index|Guest Index|  0  |
> >     -------------------------------------------------------------
> >
> > If you see there is a set of "xxxxxx" between HART and Group Indexes
> I think I'm missunderstanding the spec, as I wrote before I thought that
> [1] `Hart index` = group_idx << LHXW | hart_idx_within_the_group so,
> does the Hart Index in the schema refer to hart_idx_within_the_group or
> to [1]? The naming makes me a bit confuse.
Could you please check my comment above and if it doesn't provide answer 
to your questions I will try to explain it differently.

> > that is the reason why we have to extract HART and Group Index
> > separately as when h/w will work with target register it doesn't know
> > about "xxxxx" at all so from h/w point of view target's register hart
> > field looks like |Group Index|Hart Index|. In other words, h/w will do
> > the following with TARGET's hart index field:
> >       group_idx = hart_idx >> lhxw;
> >       hart_idx &= APLIC_xMSICFGADDR_PPN_LHX_MASK(lhxw);
> >
> > and then embed group_idx and hart_idx into the structure above.
> >
> > Does it make sense?
~ Oleksii