Re: [PATCH v7] KVM: arm64: Fix confusion in documentation for pKVM SME assert

[Marc Zyngier <maz@kernel.org>]

On Wed, 12 Feb 2025 11:11:04 +0000,
Mark Rutland <mark.rutland@arm.com> wrote:
> 
> On Wed, Feb 12, 2025 at 12:44:57AM +0000, Mark Brown wrote:
> > As raised in the review comments for the original patch the assert and
> > comment added in afb91f5f8ad7 ("KVM: arm64: Ensure that SME controls are
> > disabled in protected mode") are bogus. The comments says that we check
> > that we do not have SME enabled for a pKVM guest but the assert actually
> > checks to see if the host has anything set in SVCR which is unrelated to
> > the guest features or state, regardless of if those guests are protected
> > or not. This check is also made in the hypervisor, it will refuse to run
> > a guest if the check fails, so it appears that the assert here is
> > intended to improve diagnostics.
> > 
> > Update the comment to reflect the check in the code, and to clarify that
> > we do actually enforce this in the hypervisor. While we're here also
> > update to use a WARN_ON_ONCE() to avoid log spam if this triggers.
> > 
> > Fixes: afb91f5f8ad7 ("KVM: arm64: Ensure that SME controls are disabled in protected mode")
> > Reviewed-by: Fuad Tabba <tabba@google.com>
> > Signed-off-by: Mark Brown <broonie@kernel.org>
> > ---
> > This has been sent with v6.10 with only positive review comments after
> > the first revision, if there is some issue with the change please share
> > it.
> > 
> > To: Marc Zyngier <maz@kernel.org>
> > To: Oliver Upton <oliver.upton@linux.dev>
> > To: James Morse <james.morse@arm.com>
> > To: Suzuki K Poulose <suzuki.poulose@arm.com>
> > To: Catalin Marinas <catalin.marinas@arm.com>
> > To: Will Deacon <will@kernel.org>
> > To: Fuad Tabba <tabba@google.com>
> > Cc: Mark Rutland <mark.rutland@arm.com>
> > ---
> > Changes in v7:
> > - Reword the comment.
> > - Link to v6: https://lore.kernel.org/r/20250210-kvm-arm64-sme-assert-v6-1-cc26c46d1b43@kernel.org
> > 
> > Changes in v6:
> > - Rebase onto v6.14-rc1.
> > - Link to v5: https://lore.kernel.org/r/20241210-kvm-arm64-sme-assert-v5-1-995c8dd1025b@kernel.org
> > 
> > Changes in v5:
> > - Rebase onto v6.13-rc1.
> > - Link to v4: https://lore.kernel.org/r/20240930-kvm-arm64-sme-assert-v4-1-3c9df71db688@kernel.org
> > 
> > Changes in v4:
> > - Rebase onto v6.12-rc1
> > - Link to v3: https://lore.kernel.org/r/20240730-kvm-arm64-sme-assert-v3-1-8699454e5cb8@kernel.org
> > 
> > Changes in v3:
> > - Rebase onto v6.11-rc1.
> > - Link to v2: https://lore.kernel.org/r/20240605-kvm-arm64-sme-assert-v2-1-54391b0032f4@kernel.org
> > 
> > Changes in v2:
> > - Commit message tweaks.
> > - Change the assert to WARN_ON_ONCE().
> > - Link to v1: https://lore.kernel.org/r/20240604-kvm-arm64-sme-assert-v1-1-5d98348d00f8@kernel.org
> > ---
> >  arch/arm64/kvm/fpsimd.c | 11 +++++++----
> >  1 file changed, 7 insertions(+), 4 deletions(-)
> > 
> > diff --git a/arch/arm64/kvm/fpsimd.c b/arch/arm64/kvm/fpsimd.c
> > index 4d3d1a2eb157047b4b2488e9c4ffaabc6f5a0818..e37e53883c357093ff4455f5afdaec90e662d744 100644
> > --- a/arch/arm64/kvm/fpsimd.c
> > +++ b/arch/arm64/kvm/fpsimd.c
> > @@ -93,11 +93,14 @@ void kvm_arch_vcpu_load_fp(struct kvm_vcpu *vcpu)
> >  	}
> >  
> >  	/*
> > -	 * If normal guests gain SME support, maintain this behavior for pKVM
> > -	 * guests, which don't support SME.
> > +	 * Protected and non-protected KVM modes require that
> > +	 * SVCR.{SM,ZA} == {0,0} when entering a guest so that no
> > +	 * host/guest SME state needs to be saved/restored by hyp code.
> > +	 *
> > +	 * In protected mode, hyp code will verify this later.
> >  	 */
> > -	WARN_ON(is_protected_kvm_enabled() && system_supports_sme() &&
> > -		read_sysreg_s(SYS_SVCR));
> > +	WARN_ON_ONCE(is_protected_kvm_enabled() && system_supports_sme() &&
> > +		     read_sysreg_s(SYS_SVCR));
> 
> As I mentioned on the last round, we can drop the is_protected_kvm_enabled()
> check, i.e. have:
> 
> 	/*
> 	 * Protected and non-protected KVM modes require that
> 	 * SVCR.{SM,ZA} == {0,0} when entering a guest so that no
> 	 * host/guest SME state needs to be saved/restored by hyp code.
> 	 *
> 	 * In protected mode, hyp code will verify this later.
> 	 */
> 	WARN_ON_ONCE(system_supports_sme() && read_sysreg_s(SYS_SVCR));
> 
> Either way:
> 
> Acked-by: Mark Rutland <mark.rutland@arm.com>
> 
> Marc, are you happy to queue this atop the recent fixes from me? Those
> try to ensure SVCR.{SM,ZA} == {0,0} regardless of whether KVM is in
> protected mode.

In all honesty, I find that at this stage, the comment just gets in
the way and is over-describing what is at stake here.

The

 	WARN_ON_ONCE(system_supports_sme() && read_sysreg_s(SYS_SVCR));

is really the only thing that matters. It perfectly shows what we are
checking for, and doesn't need an exegesis.

As for the Fixes: tag, and given the magnitude of the actual fixes
that are already queued, I don't think we need it.

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.