Re: [PATCH 21/22] KVM: arm64: selftests: Test AT emulation for FEAT_HAFT

[Oliver Upton <oupton@kernel.org>]

On Tue, Jun 23, 2026 at 07:05:40PM +0000, sashiko-bot@kernel.org wrote:
> Thank you for your contribution! Sashiko AI review found 2 potential issue(s) to consider:
> - [Medium] Missing `isb()` instructions between consecutive writes to control-plane system registers.
> - [Low] Typo `sysrec_clear_set_s` instead of `sysreg_clear_set_s` causes a build failure.
> --
> 
> commit 72540780d6ff6cb4b6ed1c45760fb4372cfa7e9b
> Author: Oliver Upton <oupton@kernel.org>
> 
> KVM: arm64: selftests: Test AT emulation for FEAT_HAFT
> 
> Test that KVM's AT emulation sets the access flag on table descriptors
> when FEAT_HAFT is enabled at stage-1. Additionally, add test coverage
> that asserts the access flag is clear on table/page descriptors when the
> AT instruction generates an access flag fault.
> 
> > diff --git a/tools/testing/selftests/kvm/arm64/at.c b/tools/testing/selftests/kvm/arm64/at.c
> > index d7289f3df04fa..fb7399736f442 100644
> > --- a/tools/testing/selftests/kvm/arm64/at.c
> > +++ b/tools/testing/selftests/kvm/arm64/at.c
> 
> [ ... ]
> 
> > @@ -93,9 +103,43 @@ static void guest_code(void)
> >  	isb();
> >  	test_at(false);
> >  
> > +	if (!guest_has_haft())
> > +		GUEST_DONE();
> > +
> > +	sysrec_clear_set_s(SYS_HCRX_EL2, 0, HCRX_EL2_TCR2En);
> 
> [Severity: Low]
> There is a typo here: sysrec_clear_set_s instead of sysreg_clear_set_s.

Oops!

> > +	sysreg_clear_set_s(SYS_TCR2_EL12, 0, TCR2_EL1_HAFT);
> 
> [Severity: Medium]
> Does this sequence need an isb() between the register updates?
> 
> Updates to control-plane system registers must be synchronized with an
> explicit Context Synchronization Event before any subsequent sysreg accesses.
> Because sysreg_clear_set_s() performs reads, issuing back-to-back calls
> without an intervening isb() causes the second access to execute against an
> architecturally undefined pipeline state.

This is wrong, direct reads / writes of a system register through the
same alias appear in program order.

Thanks,
Oliver