linux-kernel - Re: [PATCH] KVM: arm64: Add missing ERX*

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Date: Tue, 16 Jan 2024 16:38:41 +0100
From: Oliver Upton <oliver.upton@...ux.dev>
To: James Morse <james.morse@....com>
Cc: Marc Zyngier <maz@...nel.org>,
	Ruidong Tian <tianruidong@...ux.alibaba.com>,
	kvmarm@...ts.linux.dev, linux-kernel@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org, suzuki.poulose@....com,
	yuzenghui@...wei.com, catalin.marinas@....com, will@...nel.org
Subject: Re: [PATCH] KVM: arm64: Add missing ERX*_EL1 registers

Hi James,

On Mon, Jan 15, 2024 at 05:21:19PM +0000, James Morse wrote:
> Hi Oliver,
> 
> On 15/01/2024 14:47, Oliver Upton wrote:
> > On Wed, Jan 10, 2024 at 12:20:30PM +0000, Marc Zyngier wrote:
> >> If my reading of the ARM ARM is correct, these registers only exist if
> >> FEAT_RASv1p1 is implemented. Which means that we shouldn't handle
> >> those as RAZ/WI unconditionally, but instead check for what we
> >> advertise to the guest and handle it accordingly.
> > 
> > Can we go a step further and just stop advertising RAS to guests? I don't
> > expect VMs to gain much from our RAZ/WI implementation.
> 
> These CPU registers would describe the error in a kernel-first setup, but firmware-first
> has its own in-memory way of doing that.
> 
> The CPU features indicates the IESB feature and ESB-instruction exist to fence errors, and
> that the CPU uses the ESR_ELx.{S,A}ET bits to describe the CPU state after an error. These
> are all useful as part of the notification of an error, be that kernel-first or
> firmware-first.
> 
> When its supported by the hardware, the VMM can inject an asynchronous external abort
> using KVM_GET_VCPU_EVENTS - otherwise the ESR_ELx.ISS bits are all imp-def, meaning all
> errors are catastrophic.
> 
> Doing this would skip save/restore of VDISR_EL2, is there any other reason to do it?

Forgive me, had the blinders on and was thinking only of the error
record interface, not ESB/DISR. In that context it makes a lot less
sense to hide RAS from guests, especially if the guest depends on ESB
being a NOP if the hardware doesn't support it.

-- 
Thanks,
Oliver