Re: [PATCH V3 21/23] xen/arm: Add mapcache invalidation handling

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Fri, 11 Dec 2020, Oleksandr wrote:
> On 11.12.20 03:28, Stefano Stabellini wrote:
> > On Thu, 10 Dec 2020, Julien Grall wrote:
> > > On 10/12/2020 02:30, Stefano Stabellini wrote:
> > > > On Mon, 30 Nov 2020, Oleksandr Tyshchenko wrote:
> > > > > From: Oleksandr Tyshchenko <oleksandr_tyshchenko@xxxxxxxx>
> > > > > 
> > > > > We need to send mapcache invalidation request to qemu/demu everytime
> > > > > the page gets removed from a guest.
> > > > > 
> > > > > At the moment, the Arm code doesn't explicitely remove the existing
> > > > > mapping before inserting the new mapping. Instead, this is done
> > > > > implicitely by __p2m_set_entry().
> > > > > 
> > > > > So we need to recognize a case when old entry is a RAM page *and*
> > > > > the new MFN is different in order to set the corresponding flag.
> > > > > The most suitable place to do this is p2m_free_entry(), there
> > > > > we can find the correct leaf type. The invalidation request
> > > > > will be sent in do_trap_hypercall() later on.
> > > > Why is it sent in do_trap_hypercall() ?
> > > I believe this is following the approach used by x86. There are actually
> > > some
> > > discussion about it (see [1]).
> > > 
> > > Leaving aside the toolstack case for now, AFAIK, the only way a guest can
> > > modify its p2m is via an hypercall. Do you have an example otherwise?
> > OK this is a very important assumption. We should write it down for sure.
> > I think it is true today on ARM.
> > 
> > 
> > > When sending the invalidation request, the vCPU will be blocked until all
> > > the
> > > IOREQ server have acknowledged the invalidation. So the hypercall seems to
> > > be
> > > the best position to do it.
> > > 
> > > Alternatively, we could use check_for_vcpu_work() to check if the mapcache
> > > needs to be invalidated. The inconvenience is we would execute a few more
> > > instructions in each entry/exit path.
> > Yeah it would be more natural to call it from check_for_vcpu_work(). If
> > we put it between #ifdef CONFIG_IOREQ_SERVER it wouldn't be bad. But I
> > am not a fan of increasing the instructions on the exit path either.
> >  From this point of view, putting it at the end of do_trap_hypercall is a
> > nice trick actually. Let's just make sure it has a good comment on top.
> > 
> > 
> > > > > Signed-off-by: Oleksandr Tyshchenko <oleksandr_tyshchenko@xxxxxxxx>
> > > > > CC: Julien Grall <julien.grall@xxxxxxx>
> > > > > 
> > > > > ---
> > > > > Please note, this is a split/cleanup/hardening of Julien's PoC:
> > > > > "Add support for Guest IO forwarding to a device emulator"
> > > > > 
> > > > > Changes V1 -> V2:
> > > > >      - new patch, some changes were derived from (+ new explanation):
> > > > >        xen/ioreq: Make x86's invalidate qemu mapcache handling common
> > > > >      - put setting of the flag into __p2m_set_entry()
> > > > >      - clarify the conditions when the flag should be set
> > > > >      - use domain_has_ioreq_server()
> > > > >      - update do_trap_hypercall() by adding local variable
> > > > > 
> > > > > Changes V2 -> V3:
> > > > >      - update patch description
> > > > >      - move check to p2m_free_entry()
> > > > >      - add a comment
> > > > >      - use "curr" instead of "v" in do_trap_hypercall()
> > > > > ---
> > > > > ---
> > > > >    xen/arch/arm/p2m.c   | 24 ++++++++++++++++--------
> > > > >    xen/arch/arm/traps.c | 13 ++++++++++---
> > > > >    2 files changed, 26 insertions(+), 11 deletions(-)
> > > > > 
> > > > > diff --git a/xen/arch/arm/p2m.c b/xen/arch/arm/p2m.c
> > > > > index 5b8d494..9674f6f 100644
> > > > > --- a/xen/arch/arm/p2m.c
> > > > > +++ b/xen/arch/arm/p2m.c
> > > > > @@ -1,6 +1,7 @@
> > > > >    #include <xen/cpu.h>
> > > > >    #include <xen/domain_page.h>
> > > > >    #include <xen/iocap.h>
> > > > > +#include <xen/ioreq.h>
> > > > >    #include <xen/lib.h>
> > > > >    #include <xen/sched.h>
> > > > >    #include <xen/softirq.h>
> > > > > @@ -749,17 +750,24 @@ static void p2m_free_entry(struct p2m_domain
> > > > > *p2m,
> > > > >        if ( !p2m_is_valid(entry) )
> > > > >            return;
> > > > >    -    /* Nothing to do but updating the stats if the entry is a
> > > > > super-page. */
> > > > > -    if ( p2m_is_superpage(entry, level) )
> > > > > +    if ( p2m_is_superpage(entry, level) || (level == 3) )
> > > > >        {
> > > > > -        p2m->stats.mappings[level]--;
> > > > > -        return;
> > > > > -    }
> > > > > +#ifdef CONFIG_IOREQ_SERVER
> > > > > +        /*
> > > > > +         * If this gets called (non-recursively) then either the
> > > > > entry
> > > > > +         * was replaced by an entry with a different base (valid
> > > > > case) or
> > > > > +         * the shattering of a superpage was failed (error case).
> > > > > +         * So, at worst, the spurious mapcache invalidation might be
> > > > > sent.
> > > > > +         */
> > > > > +        if ( domain_has_ioreq_server(p2m->domain) &&
> > > > > +             (p2m->domain == current->domain) &&
> > > > > p2m_is_ram(entry.p2m.type) )
> > > > > +            p2m->domain->mapcache_invalidate = true;
> > > > Why the (p2m->domain == current->domain) check? Shouldn't we set
> > > > mapcache_invalidate to true anyway? What happens if p2m->domain !=
> > > > current->domain? We wouldn't want the domain to lose the
> > > > mapcache_invalidate notification.
> > > This is also discussed in [1]. :) The main question is why would a
> > > toolstack/device model modify the guest memory after boot?
> > > 
> > > If we assume it does, then the device model would need to pause the domain
> > > before modifying the RAM.
> > > 
> > > We also need to make sure that all the IOREQ servers have invalidated
> > > the mapcache before the domain run again.
> > > 
> > > This would require quite a bit of work. I am not sure the effort is worth
> > > if
> > > there are no active users today.
> > OK, that explains why we think p2m->domain == current->domain, but why
> > do we need to have a check for it right here?
> > 
> > In other words, we don't think it is realistc to get here with
> > p2m->domain != current->domain, but let's say that we do somehow. What's
> > the best course of action? Probably, set mapcache_invalidate to true and
> > possibly print a warning?
> > 
> > Leaving mapcache_invalidate to false doesn't seem to be what we want to
> > do?
> > 
> >   
> > > > >        BUILD_BUG_ON(NR_hypercalls < ARRAY_SIZE(arm_hypercall_table) );
> > > > >    @@ -1459,7 +1460,7 @@ static void do_trap_hypercall(struct
> > > > > cpu_user_regs
> > > > > *regs, register_t *nr,
> > > > >            return;
> > > > >        }
> > > > >    -    current->hcall_preempted = false;
> > > > > +    curr->hcall_preempted = false;
> > > > >          perfc_incra(hypercalls, *nr);
> > > > >        call = arm_hypercall_table[*nr].fn;
> > > > > @@ -1472,7 +1473,7 @@ static void do_trap_hypercall(struct
> > > > > cpu_user_regs
> > > > > *regs, register_t *nr,
> > > > >        HYPERCALL_RESULT_REG(regs) = call(HYPERCALL_ARGS(regs));
> > > > >      #ifndef NDEBUG
> > > > > -    if ( !current->hcall_preempted )
> > > > > +    if ( !curr->hcall_preempted )
> > > > >        {
> > > > >            /* Deliberately corrupt parameter regs used by this
> > > > > hypercall.
> > > > > */
> > > > >            switch ( arm_hypercall_table[*nr].nr_args ) {
> > > > > @@ -1489,8 +1490,14 @@ static void do_trap_hypercall(struct
> > > > > cpu_user_regs
> > > > > *regs, register_t *nr,
> > > > >    #endif
> > > > >          /* Ensure the hypercall trap instruction is re-executed. */
> > > > > -    if ( current->hcall_preempted )
> > > > > +    if ( curr->hcall_preempted )
> > > > >            regs->pc -= 4;  /* re-execute 'hvc #XEN_HYPERCALL_TAG' */
> > > > > +
> > > > > +#ifdef CONFIG_IOREQ_SERVER
> > > > > +    if ( unlikely(curr->domain->mapcache_invalidate) &&
> > > > > +         test_and_clear_bool(curr->domain->mapcache_invalidate) )
> > > > > +        ioreq_signal_mapcache_invalidate();
> > > > Why not just:
> > > > 
> > > > if ( unlikely(test_and_clear_bool(curr->domain->mapcache_invalidate)) )
> > > >       ioreq_signal_mapcache_invalidate();
> > > > 
> > > This seems to match the x86 code. My guess is they tried to prevent the
> > > cost
> > > of the atomic operation if there is no chance mapcache_invalidate is true.
> > > 
> > > I am split whether the first check is worth it. The atomic operation
> > > should be
> > > uncontended most of the time, so it should be quick. But it will always be
> > > slower than just a read because there is always a store involved.
> > I am not a fun of optimizations with unclear benefits :-)
> > 
> > 
> > > On a related topic, Jan pointed out that the invalidation would not work
> > > properly if you have multiple vCPU modifying the P2M at the same time.
> > > 
> Thanks to Julien, he explained all bits in detail. Indeed I followed how it
> was done on x86 (place where to send the invalidation request, the code to
> check whether the flag is set, which at first glance, appears odd, etc)
> and review comments (to latch current into the local variable, and make sure
> that domain sends invalidation request on itself).
> Regarding what to do if p2m->domain != current->domain in p2m_free_entry().
> Probably we could set flag only if guest is paused, otherwise just print a
> warning. Thoughts?

I'd do something like:

if ( domain_has_ioreq_server(p2m->domain) && p2m_is_ram(entry.p2m.type) )
{
    WARN_ON(p2m->domain != current->domain);
    p2m->domain->mapcache_invalidate = true;
}

but maybe Julien has a better idea.