Re: [PATCH v6 02/25] x86/fpu/xstate: Fix guest fpstate allocation size calculation

[Yang, Weijiang]

On 10/21/2023 8:39 AM, Sean Christopherson wrote:
> On Thu, Sep 14, 2023, Yang Weijiang wrote:
> > Fix guest xsave area allocation size from fpu_user_cfg.default_size to
> > fpu_kernel_cfg.default_size so that the xsave area size is consistent
> > with fpstate->size set in __fpstate_reset().
> >
> > With the fix, guest fpstate size is sufficient for KVM supported guest
> > xfeatures.
> >
> > Signed-off-by: Yang Weijiang <weijiang.yang@xxxxxxxxx>
> > ---
> >   arch/x86/kernel/fpu/core.c | 4 +++-
> >   1 file changed, 3 insertions(+), 1 deletion(-)
> >
> > diff --git a/arch/x86/kernel/fpu/core.c b/arch/x86/kernel/fpu/core.c
> > index a86d37052a64..a42d8ad26ce6 100644
> > --- a/arch/x86/kernel/fpu/core.c
> > +++ b/arch/x86/kernel/fpu/core.c
> > @@ -220,7 +220,9 @@ bool fpu_alloc_guest_fpstate(struct fpu_guest *gfpu)
> >   	struct fpstate *fpstate;
> >   	unsigned int size;
> >   
> > -	size = fpu_user_cfg.default_size + ALIGN(offsetof(struct fpstate, regs), 64);
> > +	size = fpu_kernel_cfg.default_size +
> > +	       ALIGN(offsetof(struct fpstate, regs), 64);
> Shouldn't all the other calculations in this function also switch to fpu_kernel_cfg?
> At the very least, this looks wrong when paired with the above:
>
> 	gfpu->uabi_size		= sizeof(struct kvm_xsave);
> 	if (WARN_ON_ONCE(fpu_user_cfg.default_size > gfpu->uabi_size))
> 		gfpu->uabi_size = fpu_user_cfg.default_size;

Hi, Sean,
Not sure what's your concerns.
From my understanding fpu_kernel_cfg.default_size should include all enabled xfeatures in host (XCR0 | XSS),
this is also expected for supporting all guest enabled xfeatures. gfpu->uabi_size only includes enabled user
xfeatures which are operated via KVM uABIs(KVM_GET_XSAVE/KVM_SET_XSAVE/KVM_GET_XSAVE2), so the two
sizes are relatively independent since guest supervisor xfeatures are saved/restored via GET/SET_MSRS interfaces.