linux-kernel - Re: [PATCH] KVM: arm64: nv: Set ISTATUS for emulated timers, If timer expired

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Message-ID: <86eczvrz55.wl-maz@kernel.org>
Date: Tue, 18 Feb 2025 16:33:26 +0000
From: Marc Zyngier <maz@...nel.org>
To: Ganapatrao Kulkarni <gankulkarni@...amperecomputing.com>
Cc: Eric Auger <eauger@...hat.com>,
	Oliver Upton <oliver.upton@...ux.dev>,
	kvmarm <kvmarm@...ts.linux.dev>,
	linux-arm-kernel@...ts.infradead.org,
	linux-kernel@...r.kernel.org,
	christoffer.dall@....com,
	suzuki.poulose@....com,
	will@...nel.org,
	catalin.marinas@....com,
	coltonlewis@...gle.com,
	joey.gouly@....com,
	yuzenghui@...wei.com,
	darren@...amperecomputing.com,
	vishnu@...amperecomputing.com
Subject: Re: [PATCH] KVM: arm64: nv: Set ISTATUS for emulated timers, If timer expired

On Tue, 18 Feb 2025 07:33:11 +0000,
Ganapatrao Kulkarni <gankulkarni@...amperecomputing.com> wrote:
> 
> 
> Hi Marc,
> 
> On 15-02-2025 11:20 pm, Marc Zyngier wrote:
> > On Mon, 10 Feb 2025 18:26:48 +0000,
> > Eric Auger <eauger@...hat.com> wrote:
> >> 
> >> Hi Marc,
> >> 
> >> On 2/7/25 7:38 PM, Marc Zyngier wrote:
> >>> On Fri, 07 Feb 2025 18:09:58 +0000,
> >>> Oliver Upton <oliver.upton@...ux.dev> wrote:
> >>>> 
> >>>> Hey,
> >>>> 
> >>>> On Fri, Feb 07, 2025 at 05:45:33PM +0000, Marc Zyngier wrote:
> >>>>> I found at least one issue that could fail the migration. Before the
> >>>>> VM starts running, we limit the feature set to the subset we actually
> >>>>> support with NV.
> >>>>> 
> >>>>> By doing this, we also change the value of IDreg fields that are not
> >>>>> writable, because they describe features that we don't support.
> >>>>> Obviously, that fails on restore.
> >>>>> 
> >>>>> I need to have a think...
> >>>> 
> >>>> We spoke about this a while ago (and I forgot til now), but I was
> >>>> wondering if we could use vCPU feature flags to describe NV, including
> >>>> the selection between FEAT_E2H0 and FEAT_VHE.
> >>>> 
> >>>> I think this might match userspace expectations a bit more closely where
> >>>> the state of the ID registers after init gives the actual feature set
> >>>> supported by the VM.
> >>> 
> >>> I'm not sure that's enough. Let me give you an example:
> >>> 
> >>> My host has FEAT_XNX, described in ID_AA64MMFR1_EL1.XNX. For whatever
> >>> reason, we don't allow this field to be written to, even out of NV
> >>> context. This is odd, because for an EL1 VM, this field means nothing
> >>> at all.
> >> So the curprit fields for me look like
> >> 
> >> - ID_AA64MMFR1_EL1.XNX
> >> - ID_AA64DFR0_EL1.DoubleLock
> >> - ID_AA64PFR0_EL1.RAS
> >> 
> >> This is still based on your nv-next branch from Jan 9
> >> https://github.com/eauger/linux/tree/nv_next_jan9_2025
> > 
> > I have now pushed out a new nv-next branch with the new and improved
> > UAPI. I expect migration to work a bit better, or at least not to
> > explode on ID register restore. You will notice that things have
> > changed a bit (extra flag and cap for FEAT_E2H0), but nothing really
> > major.
> > 
> 
> Tried nv-next branch and it is breaking(kernel Oops) for normal VM
> boot itself with qemu. Looks like this is happening since qemu is
> trying to write to ID_UNALLOCATED mapped registers as part of
> save-restore of registers.

Yeah, ID_UNALLOCATED is in pretty bad shape overall. For start, it
doesn't even have a name associated to it, and then everything else
falls apart.

I'll rework it shortly, thanks for the heads up.

	M.

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