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

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Date:   Tue, 24 Oct 2023 09:32:23 -0700
From:   Sean Christopherson <seanjc@...gle.com>
To:     Weijiang Yang <weijiang.yang@...el.com>
Cc:     pbonzini@...hat.com, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, dave.hansen@...el.com,
        peterz@...radead.org, chao.gao@...el.com,
        rick.p.edgecombe@...el.com, john.allen@....com
Subject: Re: [PATCH v6 02/25] x86/fpu/xstate: Fix guest fpstate allocation
 size calculation

On Tue, Oct 24, 2023, Weijiang Yang wrote:
> 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@...el.com>
> > > ---
> > >   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.

Ah, right, I keep forgetting that KVM's ABI can't use XRSTOR because it forces
the compacted format.

This part still looks odd to me:

	gfpu->xfeatures		= fpu_user_cfg.default_features;
	gfpu->perm		= fpu_user_cfg.default_features;

but I'm probably just not understanding something in the other patches changes yet.