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

[Marc Zyngier <maz@kernel.org>]

On Mon, 25 Mar 2024 00:27:43 +0000,
Mark Brown <broonie@kernel.org> wrote:
> 
> > >  - 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).

But what behaviour do you expect from allowing such a write? Insert in
place? Or zero the upper bits of the vector, as per R_WKYLB? One is
wrong, and the other wrecks havoc on unsuspecting userspace.

My take on this is that when a VM is S*E aware, only the writes to the
largest *enabled* registers should take place. This is similar to what
we do for FP/SIMD: we only allow writes to the V registers, and not to
Q, D, S, H or B, although that happens by construction. For S*E,
dropping the write on the floor (or return some error that userspace
will understand as benign) is the least bad option.

	M.

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