linux-kernel - Re: [PATCH 4/4] arm64: mte: Use stage-2 NoTagAccess memory attribute if supported

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Message-ID: <Zx-oyZWF12M_ka-m@linux.dev>
Date: Mon, 28 Oct 2024 15:07:53 +0000
From: Oliver Upton <oliver.upton@...ux.dev>
To: "Aneesh Kumar K.V" <aneesh.kumar@...nel.org>
Cc: linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	kvmarm@...ts.linux.dev, Suzuki K Poulose <Suzuki.Poulose@....com>,
	Steven Price <steven.price@....com>, Will Deacon <will@...nel.org>,
	Catalin Marinas <catalin.marinas@....com>,
	Marc Zyngier <maz@...nel.org>, Mark Rutland <mark.rutland@....com>,
	Joey Gouly <joey.gouly@....com>, Zenghui Yu <yuzenghui@...wei.com>
Subject: Re: [PATCH 4/4] arm64: mte: Use stage-2 NoTagAccess memory attribute
 if supported

On Mon, Oct 28, 2024 at 08:22:02PM +0530, Aneesh Kumar K.V wrote:
> 
> Hi Oliver,
> 
> 
> Thanks for reviewing the changes.
> 
> Oliver Upton <oliver.upton@...ux.dev> writes:
> 
> > On Mon, Oct 28, 2024 at 03:10:14PM +0530, Aneesh Kumar K.V (Arm) wrote:
> >
> 
> > NOTE: We could also use KVM_EXIT_MEMORY_FAULT for this. I chose to
> > add a new EXIT type because this is arm64 specific exit type.
> >
> > Signed-off-by: Aneesh Kumar K.V (Arm) <aneesh.kumar@...nel.org>
> > ---
> 
> I have used KVM_EXIT_MEMORY_FAULT as part of the initial prototype.

Ah, apologies, I clearly didn't read the changelog.

> >> +		/* KVM_EXIT_ARM_NOTAG_ACCESS */
> >> +		struct {
> >> +			__u64 flags;
> >> +			__u64 gpa;
> >> +			__u64 size;
> >> +		} notag_access;
> >
> > Can you please look into reusing the memory fault exit infrastructure?
> >
> > The entire point of that is for KVM to tell the VMM it cannot make
> > forward progress because of ${SOMETHING} unexpected at the specified
> > GPA. You can add a new flag that describes tag access.
> >
> 
> The only reason I dropped the change was because the flag will be very much
> arm64 specific.

Eh, making it arm64-specific is very much the right call. There's no
shortage of available bits in that structure, and we should make no
attempt to provide a generic description of what is otherwise a very
architecture-specific thing.

> Based on your feedback, I will switch to KVM_EXIT_MEMORY_FAULT in the next
> update.

Excellent, thanks!

-- 
Thanks,
Oliver