Re: [PATCH V8 1/2] KVM: selftests: Add x86_64 guest udelay() utility

[Sean Christopherson <seanjc@google.com>]

On Tue, Jun 11, 2024, Reinette Chatre wrote:
> > Heh, the docs are stale.  KVM hasn't returned an error since commit cc578287e322
> > ("KVM: Infrastructure for software and hardware based TSC rate scaling"), which
> > again predates selftests by many years (6+ in this case).  To make our lives
> > much simpler, I think we should assert that KVM_GET_TSC_KHZ succeeds, and maybe
> > throw in a GUEST_ASSERT(thz_khz) in udelay()?
> 
> I added the GUEST_ASSERT() but I find that it comes with a caveat (more below).
> 
> I plan an assert as below that would end up testing the same as what a
> GUEST_ASSERT(tsc_khz) would accomplish:
> 
> 	r = __vm_ioctl(vm, KVM_GET_TSC_KHZ, NULL);
> 	TEST_ASSERT(r > 0, "KVM_GET_TSC_KHZ did not provide a valid TSC freq.");
> 	tsc_khz = r;
> 
> 
> Caveat is: the additional GUEST_ASSERT() requires all tests that use udelay() in
> the guest to now subtly be required to implement a ucall (UCALL_ABORT) handler.
> I did a crude grep check to see and of the 69 x86_64 tests there are 47 that do
> indeed have a UCALL_ABORT handler. If any of the other use udelay() then the
> GUEST_ASSERT() will of course still trigger, but will be quite cryptic. For
> example, "Unhandled exception '0xe' at guest RIP '0x0'" vs. "tsc_khz".

Yeah, we really need to add a bit more infrastructure, there is way, way, waaaay
too much boilerplate needed just to run a guest and handle the basic ucalls.
Reporting guest asserts should Just Work for 99.9% of tests.

Anyways, is it any less cryptic if ucall_assert() forces a failure?  I forget if
the problem with an unhandled GUEST_ASSERT() is that the test re-enters the guest,
or if it's something else.

I don't think we need a perfect solution _now_, as tsc_khz really should never
be 0, just something to not make life completely miserable for future developers.

diff --git a/tools/testing/selftests/kvm/lib/ucall_common.c b/tools/testing/selftests/kvm/lib/ucall_common.c
index 42151e571953..1116bce5cdbf 100644
--- a/tools/testing/selftests/kvm/lib/ucall_common.c
+++ b/tools/testing/selftests/kvm/lib/ucall_common.c
@@ -98,6 +98,8 @@ void ucall_assert(uint64_t cmd, const char *exp, const char *file,
 
        ucall_arch_do_ucall((vm_vaddr_t)uc->hva);
 
+       ucall_arch_do_ucall(GUEST_UCALL_FAILED);
+
        ucall_free(uc);
 }