Re: [PATCH v2 10/11] KVM: arm64: Save/restore SVE state for nVHE

From: Marc Zyngier <maz@kernel.org>
To: Will Deacon <will@kernel.org>
Cc: kvmarm@lists.cs.columbia.edu,
	linux-arm-kernel@lists.infradead.org, kvm@vger.kernel.org,
	dave.martin@arm.com, daniel.kiss@arm.com,
	Catalin Marinas <catalin.marinas@arm.com>,
	James Morse <james.morse@arm.com>,
	Julien Thierry <julien.thierry.kdev@gmail.com>,
	Suzuki K Poulose <suzuki.poulose@arm.com>,
	broonie@kernel.org, ascull@google.com, qperret@google.com,
	kernel-team@android.com
Subject: Re: [PATCH v2 10/11] KVM: arm64: Save/restore SVE state for nVHE
Date: Thu, 18 Mar 2021 14:32:27 +0000	[thread overview]
Message-ID: <87zgz0h6ro.wl-maz@kernel.org> (raw)
In-Reply-To: <20210318141358.GF7055@willie-the-truck>

On Thu, 18 Mar 2021 14:13:58 +0000,
Will Deacon <will@kernel.org> wrote:
> 
> On Thu, Mar 18, 2021 at 12:25:31PM +0000, Marc Zyngier wrote:
> > Implement the SVE save/restore for nVHE, following a similar
> > logic to that of the VHE implementation:
> > 
> > - the SVE state is switched on trap from EL1 to EL2
> > 
> > - no further changes to ZCR_EL2 occur as long as the guest isn't
> >   preempted or exit to userspace
> > 
> > - ZCR_EL2 is reset to its default value on the first SVE access from
> >   the host EL1, and ZCR_EL1 restored to the default guest value in
> >   vcpu_put()
> > 
> > Signed-off-by: Marc Zyngier <maz@kernel.org>
> > ---
> >  arch/arm64/kvm/fpsimd.c                 | 10 +++++--
> >  arch/arm64/kvm/hyp/include/hyp/switch.h | 37 +++++++++----------------
> >  arch/arm64/kvm/hyp/nvhe/switch.c        |  4 +--
> >  3 files changed, 23 insertions(+), 28 deletions(-)
> > 
> > diff --git a/arch/arm64/kvm/fpsimd.c b/arch/arm64/kvm/fpsimd.c
> > index 14ea05c5134a..5621020b28de 100644
> > --- a/arch/arm64/kvm/fpsimd.c
> > +++ b/arch/arm64/kvm/fpsimd.c
> > @@ -121,11 +121,17 @@ void kvm_arch_vcpu_put_fp(struct kvm_vcpu *vcpu)
> >  	local_irq_save(flags);
> >  
> >  	if (vcpu->arch.flags & KVM_ARM64_FP_ENABLED) {
> > -		if (guest_has_sve)
> > +		if (guest_has_sve) {
> >  			__vcpu_sys_reg(vcpu, ZCR_EL1) = read_sysreg_el1(SYS_ZCR);
> >  
> > +			/* Restore the VL that was saved when bound to the CPU */
> > +			if (!has_vhe())
> > +				sve_cond_update_zcr_vq(vcpu_sve_max_vq(vcpu) - 1,
> > +						       SYS_ZCR_EL1);
> 
> You end up reading ZCR_EL1 twice here, but it's probably not the end of the
> world.

True, but I'd expect read accesses to ZCR_EL1 to be reasonably fast,
something we can't expect from writes.

> 
> Anyway:
> 
> Acked-by: Will Deacon <will@kernel.org>

Thanks,

	M.

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

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