Re: [PATCH v2 5/5] KVM: arm64: Exclude FP ownership from kvm_vcpu_arch

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On Sat, Mar 23, 2024 at 07:06:24PM +0000, Marc Zyngier wrote:
> Mark Brown <broonie@xxxxxxxxxx> wrote:

> > There's still the thing with the interaction with SME support - to
> > summarise what I think you're asking for the userspace ABI there:

> Well, the SME support is still pretty prospective, and this patch has
> no impact on an existing ABI.

Sure, hopefully I should have a new verison out this release - it was
mostly held up waiting for the ID register parsing framework you got
merged last release.  Though holidays do make things a bit tight timing

> >  - Add support for the V registers in the sysreg interface when SVE is
> >    enabled.

> We already support the V registers with KVM_REG_ARM_CORE_REG(). Why
> would you add any new interface for this?  The kernel should be
> perfectly capable of dealing with the placement of the data in the
> internal structures, and there is no need to tie the userspace ABI to
> how we deal with that placement (kvm_regs is already purely
> userspace).

This was referring to the fact that currently when SVE is enabled access
to the V registers as V registers via _CORE_REG() is blocked and they
can only be accessed as a subset of the Z registers (see the check at
the end of core_reg_size_from_offset() in guest.c).

> > then the implementation can do what it likes to achieve that, the most
> > obvious thing being to store in native format for the current hardware
> > mode based on SVCR.{SM,ZA}.  Does that sound about right?

> Apart from the statement about the V registers, this seems OK. But
> again, I want to see this agreed with the QEMU folks.

Great, thanks.

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