Re: [PATCH v1 6/6] xen/riscv: enable DOMAIN_BUILD_HELPERS

[Oleksii Kurochko <oleksii.kurochko@xxxxxxxxx>]

On 3/19/26 8:58 AM, Jan Beulich wrote:
> On 17.03.2026 13:49, Oleksii Kurochko wrote:
> > On 2/13/26 2:11 PM, Jan Beulich wrote:
> > > > > > +#define GUEST_RAM0_BASE   xen_mk_ullong(0x80000000) /* 2GB of low RAM @ 2GB */
> > > > > > +#define GUEST_RAM0_SIZE   xen_mk_ullong(0x80000000)
> > > > > > +
> > > > > > +#define GUEST_RAM_BANK_BASES   { GUEST_RAM0_BASE }
> > > > > > +#define GUEST_RAM_BANK_SIZES   { GUEST_RAM0_SIZE }
> > (cut)
> >
> > > If all you want are 2Gb guests, why would such guests be 64-bit? And with
> > > (iirc) RV32 permitting more than 4Gb (via PPN being 22 bits wide), perhaps
> > > even a 32-bit hypervisor would suffice?
> > Btw, shouldn't we look at VPN width?
> >
> > My understanding is that we should take GUEST_RAM0_BASE as sgfn address
> > and then map it to mfn's page (allocated by alloc_domheap_pages())? And then
> > repeat this process until we won't map GUEST_RAM0_SIZE.
> >
> > In this case for RV32 VPN (which is GFN in the current context) is 32-bit
> > wide as RV32 supports only Sv32, what is 2^32 - 1, what is almost 4gb.
> ??? (IOW - I fear I'm confused enough by the question that I don't know how
> to respond.)
You mentioned above that:
  "... And with (iirc) RV32 permitting more than 4Gb (via PPN being 22 bits wide) 
..."

I wanted to clarify why you use PPN here in the context of GUEST_RAM0_BASE 
definition.
(and maybe I just misinterpreted incorrectly your original message)
GUEST_RAM0_BASE is the address at which the guest believes RAM starts in its 
physical
address space, i.e. it is a GPA, which is then translated to an MPA.

From the MMU's perspective, the GPA looks like:
  VPN[1] | VPN[0] | page_offset   (in Sv32x4 mode)

In Sv32x4, the GPA is 34 bits wide (or 22 bits wide in terms of GFNs), and the 
MPA is
also 32 bits wide (or 22 bits wide in terms of PPN).

The distinction is not significant in Sv32x4, since PPN width equals VPN width, 
but
in other modes VPN < PPN (in terms of bit width).
So when we want to run a guest in Sv39x4 mode and want to give the guest the 
full
Sv39x4 address space, setting GUEST_RAM0_SIZE to the maximum possible value for
Sv39x4, shouldn't we look at the VPN width rather than the PPN width?
In other words, GUEST_RAM0_SIZE should be (2^41 - 1) rather than (2^56 - 1)
for Sv39x4.

~ Oleksii