From: Mark Rutland <mark.rutland@arm•com>
To: Mark Brown <broonie@kernel•org>
Cc: Marc Zyngier <maz@kernel•org>, Oliver Upton <oupton@kernel•org>,
Joey Gouly <joey.gouly@arm•com>,
Steffen Eiden <seiden@linux•ibm.com>,
Suzuki K Poulose <suzuki.poulose@arm•com>,
Catalin Marinas <catalin.marinas@arm•com>,
Will Deacon <will@kernel•org>,
linux-arm-kernel@lists•infradead.org, kvmarm@lists•linux.dev,
linux-kernel@vger•kernel.org
Subject: Re: [PATCH] KVM: arm64: Preserve all guest ZCR_EL2.LEN values
Date: Mon, 25 May 2026 19:36:50 +0100 [thread overview]
Message-ID: <ahSWwl-uCv4ZW-nE@J2N7QTR9R3> (raw)
In-Reply-To: <20260522-kvm-arm64-fix-zcr-len-nv-v1-1-ec254e9078cf@kernel.org>
On Fri, May 22, 2026 at 07:00:04PM +0100, Mark Brown wrote:
> Since b3d29a823099 ("KVM: arm64: nv: Handle ZCR_EL2 traps") when guests
> write to ZCR_EL2 we have clamped the value of ZCR_EL2.LEN to be at most
> that configuring the maximum guest VL. This is not the behaviour the
> architecture documents for ZCR_EL2.LEN, the expectation is that all bits
> will be read as written. Further, writing values larger than the largest
> available vector length is part of the documented procedure for enumerating
> the supported vector lengths so we expect to see this happen in practice.
>
> The reasoning for the current behaviour is not specifically articulated, my
> best guess is that it is intended to ensure that the guest can not see an
> effective VL greater than the maximum that has been configured. This can
> instead be achieved by configuring ZCR_EL2 when loading guest state:
>
> - When running at EL0 or EL1 configure ZCR_EL2.LEN to the minimum of the
> guest ZCR_EL2.LEN and vcpu_sve_max_vq(vcpu)-1.
> - When running at EL2 configure the maximum VL for the guest in
> ZCR_EL2.LEN like we do for non-nested guests and load the guest
> ZCR_EL2 into ZCR_EL1.
>
> This will ensure that the guest sees both the ZCR_EL2.LEN value which it
> wrote and the effective VL that resulting from the values it has configured
> in ZCR_ELx.LEN.
>
> Currently all other bits in ZCR_EL2 are either RES0 or RAZ/WI, values
> written are sanitised based on this.
>
> Fixes: b3d29a823099 ("KVM: arm64: nv: Handle ZCR_EL2 traps")
> Signed-off-by: Mark Brown <broonie@kernel•org>
For context, I mentioned this potential problem to Mark, and described
this possible solution at:
https://lore.kernel.org/linux-arm-kernel/af4bWxiOogfPz_dp@J2N7QTR9R3/
I said:
AFAICT, none of the values for the SMCR_ELx.LEN and ZCR_ELx.LEN fields
are reserved or unallocated. Thus all the bits of those fields should
be stateful, and a read should observe the last value written,
regardless of the effective value of the field.
[...]
Either what we're doing is wrong, or the architcture requires a
clarification to say that values corresponding to unimplmented vector
lengths are reserved.
Have we sought feedback from architects? While I said "*or* the
architcture requires a clarification", I think it should be clarified
more explicitly either way given that the pattern is unusual.
Given this is particularly subtle, please keep me in the loop when
speaking with architects about this.
Mark.
next prev parent reply other threads:[~2026-05-25 18:37 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2026-05-22 18:00 [PATCH] KVM: arm64: Preserve all guest ZCR_EL2.LEN values Mark Brown
2026-05-23 8:47 ` Marc Zyngier
2026-05-23 14:38 ` Mark Brown
2026-05-23 15:24 ` Marc Zyngier
2026-05-25 18:36 ` Mark Rutland [this message]
2026-05-26 10:19 ` Mark Brown
2026-05-26 12:03 ` Mark Rutland
2026-05-26 13:03 ` Mark Brown
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=ahSWwl-uCv4ZW-nE@J2N7QTR9R3 \
--to=mark.rutland@arm$(echo .)com \
--cc=broonie@kernel$(echo .)org \
--cc=catalin.marinas@arm$(echo .)com \
--cc=joey.gouly@arm$(echo .)com \
--cc=kvmarm@lists$(echo .)linux.dev \
--cc=linux-arm-kernel@lists$(echo .)infradead.org \
--cc=linux-kernel@vger$(echo .)kernel.org \
--cc=maz@kernel$(echo .)org \
--cc=oupton@kernel$(echo .)org \
--cc=seiden@linux$(echo .)ibm.com \
--cc=suzuki.poulose@arm$(echo .)com \
--cc=will@kernel$(echo .)org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox