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

[Ganapatrao Kulkarni]

On 19-02-2025 02:54 am, Marc Zyngier wrote:
> On Tue, 18 Feb 2025 07:33:11 +0000,
> Ganapatrao Kulkarni <gankulkarni@xxxxxxxxxxxxxxxxxxxxxx> 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@xxxxxxxxxx> 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@xxxxxxxxx> 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.
>
> My take on this problem ends up being more consolidation, and make
> sure that the individual macros only override the default callbacks
> for idregs.
>
> Additionally, ID_UNALLOCATED gets a name matching the architectural
> encoding.

Thanks, having name helps future debugging.
While I was debugging, the register name was showing as null and I have 
to map id to encodings.

I have rebased to nv-next and looks all good!.

--
Thanks,
Ganapat/GK