Re: [kvm PATCH v7 2/2] kvm: x86: Dynamically allocate guest_fpu

[Marc Orr <marcorr@google.com>]

On Fri, Dec 21, 2018 at 2:28 AM Paolo Bonzini <pbonzini@redhat.com> wrote:
>
> On 06/11/18 23:20, Marc Orr wrote:
> > +     x86_fpu_cache = kmem_cache_create_usercopy(
> > +                             "x86_fpu",
> > +                             fpu_kernel_xstate_size,
>
> This unfortunately is wrong because there are other members in struct
> fpu before the fpregs_state union.  It's enough to run a guest and then
> rmmod kvm to see slub errors which are actually caused by memory
> corruption.
>
> The right way to size it is shown in fpu__init_task_struct_size but for
> now I'll revert it to sizeof(struct fpu).  I have plans to move
> fsave/fxsave/xsave directly in KVM, without using the kernel FPU
> helpers, and actually this guest_fpu thing will come in handy for that.
> :)  Once it's done, the size of the object in the cache will be
> something like kvm_xstate_size.
>
> Paolo
>
>
> > +                             __alignof__(struct fpu),
> > +                             SLAB_ACCOUNT,
> > +                             offsetof(struct fpu, state),
> > +                             fpu_kernel_xstate_size,
> > +                             NULL);
>

Oops. Thanks for debugging, explaining and fixing!