Re: [PATCH 2/4] KVM: selftests: Setup ucall after loading program into guest memory

Sean Christopherson
2022-12-09T01:09:01Z

On Thu, Dec 08, 2022, Ricardo Koller wrote:
> On Thu, Dec 08, 2022 at 07:01:57PM +0000, Sean Christopherson wrote:
> > On Thu, Dec 08, 2022, Ricardo Koller wrote:
> > > On Thu, Dec 08, 2022 at 12:37:23AM +0000, Oliver Upton wrote:
> > > > On Thu, Dec 08, 2022 at 12:24:20AM +0000, Sean Christopherson wrote:
> > > > > > Even still, that's just a kludge to make ucalls work. We have other
> > > > > > MMIO devices (GIC distributor, for example) that work by chance since
> > > > > > nothing conflicts with the constant GPAs we've selected in the tests.
> > > > > >
> > > > > > I'd rather we go down the route of having an address allocator for the
> > > > > > for both the VA and PA spaces to provide carveouts at runtime.
> > > > >
> > > > > Aren't those two separate issues? The PA, a.k.a. memslots space, can be solved
> > > > > by allocating a dedicated memslot, i.e. doesn't need a carve. At worst, collisions
> > > > > will yield very explicit asserts, which IMO is better than whatever might go wrong
> > > > > with a carve out.
> > > >
> > > > Perhaps the use of the term 'carveout' wasn't right here.
> > > >
> > > > What I'm suggesting is we cannot rely on KVM memslots alone to act as an
> > > > allocator for the PA space. KVM can provide devices to the guest that
> > > > aren't represented as memslots. If we're trying to fix PA allocations
> > > > anyway, why not make it generic enough to suit the needs of things
> > > > beyond ucalls?
> > >
> > > One extra bit of information: in arm, IO is any access to an address (within
> > > bounds) not backed by a memslot. Not the same as x86 where MMIO are writes to
> > > read-only memslots. No idea what other arches do.
> >
> > I don't think that's correct, doesn't this code turn write abort on a RO memslot
> > into an io_mem_abort()? Specifically, the "(write_fault && !writable)" check will
> > match, and assuming none the the edge cases in the if-statement fire, KVM will
> > send the write down io_mem_abort().
>
> You are right. In fact, page_fault_test checks precisely that: writes on
> RO memslots are sent to userspace as an mmio exit. I was just referring
> to the MMIO done for ucall.

To clarify for others, Ricardo thought that x86 selftests were already using a
read-only memslot for ucalls, hence the confusion.

> Having said that, we could use ucall as writes on read-only memslots
> like what x86 does.

+1. x86 currently uses I/O with a hardcoded port, but theoretically that's just
as error prone as hardcoding a GPA, it just works because x86 doesn't have any
port I/O tests.

Ugh, and that made me look at sync_regs_test.c, which does its own open coded
ucall. That thing is probably working by dumb luck at this point.