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

From: Sean Christopherson <seanjc@google.com>
To: Ricardo Koller <ricarkol@google.com>
Cc: Shuah Khan <shuah@kernel.org>,
	kvm@vger.kernel.org, Marc Zyngier <maz@kernel.org>,
	linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org,
	kvmarm@lists.linux.dev, Paolo Bonzini <pbonzini@redhat.com>,
	kvmarm@lists.cs.columbia.edu,
	linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH 2/4] KVM: selftests: Setup ucall after loading program into guest memory
Date: Fri, 9 Dec 2022 01:08:53 +0000	[thread overview]
Message-ID: <Y5KKpQSd8H88vDoH@google.com> (raw)
In-Reply-To: <Y5I/xiFMLVbpAZj+@google.com>

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.
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