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From: Marc Zyngier <maz@kernel.org>
To: Andrew Scull <ascull@google.com>
Cc: kvmarm@lists.cs.columbia.edu,
	linux-arm-kernel@lists.infradead.org, kvm@vger.kernel.org,
	dave.martin@arm.com, daniel.kiss@arm.com,
	Will Deacon <will@kernel.org>,
	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, kernel-team@android.com
Subject: Re: [PATCH 08/10] KVM: arm64: Add a nVHE-specific SVE VQ reset hypercall
Date: Tue, 16 Mar 2021 15:00:45 +0000	[thread overview]
Message-ID: <875z1rf8iq.wl-maz@kernel.org> (raw)
In-Reply-To: <YFC/pmzqSuXq+3+I@google.com>

On Tue, 16 Mar 2021 14:24:38 +0000,
Andrew Scull <ascull@google.com> wrote:
> 
> On Tue, Mar 16, 2021 at 10:13:10AM +0000, Marc Zyngier wrote:
> > ZCR_EL2 controls the upper bound for ZCR_EL1, and is set to
> > a potentially lower limit when the guest uses SVE.
> > 
> > In order to restore the SVE state on the EL1 host, we must first
> > reset ZCR_EL2 to its original value.
> > 
> > Provide a hypervall that perform this reset.
> 
> Is there a good reason to have an explicit hypercall vs trapping the
> host access to SVE and restoring in that event?

Trapping ZCR_EL2 isn't possible, as it would UNDEF at EL1. Trapping
ZCR_EL1 accesses is possible though, but it'd mean leaving the SVE
traps enabled on guest exit, something we currently don't do.

> It's quite easy to do trap handling at EL2 now and it could let things
> be even lazier, if that's any benefit in this case.

We don't really have a good infrastructure for dealing with individual
sysregs (pKVM will eventually change that, but we're not there yet,
and it isn't clear how that'd apply to non-protected), so we'd have to
deal with the whole SVE EC.

What we could do is:

- set CPTR_EL2.TZ set on guest exit

- on SVE trap, reset ZCR_EL2, clear CPTR_EL2.TZ, reexecute the
  faulting instruction.

I can have a look at how badly it looks.

> Trapping seems to have had a bad rep in other conversations but I'm not
> sure the same reasoning applies to this as well, or not.

HVC and traps have the same basic cost. I seriously doubt you can
measure the difference on any real CPU.

Thanks,

	M.

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

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  reply	other threads:[~2021-03-16 15:02 UTC|newest]

Thread overview: 31+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-03-16 10:13 [PATCH 00/10] KVM: arm64: Enable SVE support on nVHE systems Marc Zyngier
2021-03-16 10:13 ` [PATCH 01/10] KVM: arm64: Provide KVM's own save/restore SVE primitives Marc Zyngier
2021-03-16 10:31   ` Quentin Perret
2021-03-16 12:17     ` Marc Zyngier
2021-03-17 14:30   ` Will Deacon
2021-03-16 10:13 ` [PATCH 02/10] KVM: arm64: Use {read, write}_sysreg_el1 to access ZCR_EL1 Marc Zyngier
2021-03-17 14:31   ` [PATCH 02/10] KVM: arm64: Use {read,write}_sysreg_el1 " Will Deacon
2021-03-16 10:13 ` [PATCH 03/10] KVM: arm64: Let vcpu_sve_pffr() handle HYP VAs Marc Zyngier
2021-03-17 14:31   ` Will Deacon
2021-03-16 10:13 ` [PATCH 04/10] KVM: arm64: Introduce vcpu_sve_vq() helper Marc Zyngier
2021-03-17 14:01   ` Will Deacon
2021-03-16 10:13 ` [PATCH 05/10] KVM: arm64: Rework SVE host-save/guest-restore Marc Zyngier
2021-03-17 14:29   ` Will Deacon
2021-03-17 14:54     ` Marc Zyngier
2021-03-16 10:13 ` [PATCH 06/10] KVM: arm64: Map SVE context at EL2 when available Marc Zyngier
2021-03-17 16:01   ` Will Deacon
2021-03-18  8:56     ` Marc Zyngier
2021-03-16 10:13 ` [PATCH 07/10] KVM: arm64: Save guest's ZCR_EL1 before saving the FPSIMD state Marc Zyngier
2021-03-17 17:17   ` Will Deacon
2021-03-17 17:20     ` Will Deacon
2021-03-16 10:13 ` [PATCH 08/10] KVM: arm64: Add a nVHE-specific SVE VQ reset hypercall Marc Zyngier
2021-03-16 10:45   ` Quentin Perret
2021-03-16 12:18     ` Marc Zyngier
2021-03-16 14:24   ` Andrew Scull
2021-03-16 15:00     ` Marc Zyngier [this message]
2021-03-16 10:13 ` [PATCH 09/10] KVM: arm64: Save/restore SVE state for nVHE Marc Zyngier
2021-03-17 17:57   ` Will Deacon
2021-03-18  9:12     ` Marc Zyngier
2021-03-16 10:13 ` [PATCH 10/10] KVM: arm64: Enable SVE support " Marc Zyngier
2021-03-17 18:00   ` Will Deacon
2021-03-18  9:14     ` Marc Zyngier

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