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

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Message-ID: <af972fe5981b9e7101b64de43c7be0a8cc165323.camel@redhat.com>
Date:   Tue, 31 Oct 2023 19:43:54 +0200
From:   Maxim Levitsky <mlevitsk@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        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, 2023-10-24 at 09:32 -0700, Sean Christopherson wrote:
> 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;

That should be indeed fpu_kernel_cfg.default_features. 
This variable is also currently hardly used, it only tracks which dynamic userspace features
are enabled and KVM only uses it once (in fpu_enable_guest_xfd_features)



> 	gfpu->perm		= fpu_user_cfg.default_features;


This variable I think is currently only set and never read.

Note that current->group_leader->thread.fpu.guest_perm is actually initialized to fpu_kernel_cfg.default_features
but the kernel components of it masked in the corresponding prctl 
(ARCH_GET_XCOMP_SUPP/ARCH_GET_XCOMP_GUEST_PERM/ARCH_REQ_XCOMP_GUEST_PERM).

So I think that we also should use fpu_kernel_cfg.default_features here for the sake of not having uninitilized
variable on 32 bit kernels, because the whole FPU permission thing I see is implemented for 64 bit kernels only.

Or even better IMHO is to remove both variables and in fpu_enable_guest_xfd_features,
just mask the xfeatures with the XFEATURE_MASK_USER_DYNAMIC instead.


Best regards,
	Maxim Levitsky

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