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    SubjectRE: [PATCH v5 07/21] x86/fpu: Provide fpu_enable_guest_xfd_features() for KVM
    Date
    > From: Paolo Bonzini <pbonzini@redhat.com>
    > Sent: Wednesday, January 5, 2022 9:07 PM
    >
    > On 1/5/22 13:35, Yang Zhong wrote:
    > > +int fpu_enable_guest_xfd_features(struct fpu_guest *guest_fpu, u64
    > xfeatures)
    > > +{
    > > + lockdep_assert_preemption_enabled();
    > > +
    >
    > The old fpu_update_guest_perm_features(guest_fpu) is equivalent to
    >
    > fpu_enable_guest_xfd_features(guest_fpu, guest_fpu->perm);
    >
    > Missing doc comment:
    >
    > /*
    > * fpu_enable_guest_xfd_features - Enable xfeatures according to guest
    > perm
    > * @guest_fpu: Pointer to the guest FPU container
    > * @xfeatures: Features requested by guest CPUID
    > *
    > * Enable all dynamic xfeatures according to guest perm and requested
    > CPUID.
    > * Invoked if the caller wants to conservatively expand fpstate buffer instead
    > * of waiting until XCR0 or XFD MSR is written.
    > *
    > * Return: 0 on success, error code otherwise
    > */

    It's not equivalent. The old interface enables all xfeatures allowed by
    guest perm while the new one just enables feature bits according to
    the caller request. It also becomes a more general interface instead of
    being only for conservative expansion. Since both points in the old
    comment don't hold now and this function is obvious, we didn't put
    a comment here (on par with other KVM helpers in that block).

    If still necessary we could add one like below:

    /*
    * fpu_enable_guest_xfd_features - Enable xfeatures for guest fpu container
    * @guest_fpu: Pointer to the guest FPU container
    * @xfeatures: Features requested by the caller
    *
    * Enable dynamic xfeatures and expand guest fpstate buffer accordingly.
    * KVM should call this function before the requested xfeatures are used
    * by the guest.
    *
    * Return: 0 on success, error code otherwise
    */

    >
    > Also, the check for 32-bit is slightly imprecise:
    >
    > /* Dynamic xfeatures are not supported with 32-bit kernels. */
    > if (!IS_ENABLED(CONFIG_X86_64))
    > - return 0;
    > + return -EINVAL;
    >
    > since we only get here with xfeatures != 0 (if it compiles, just removing
    > the IS_ENABLED check altogether would be even better). With these changes,
    >
    > Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
    >
    > Thanks,
    >
    > Paolo

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