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    SubjectRe: [patch 0/6] x86/fpu: Preparatory changes for guest AMX support
    Date
    On Tue, Dec 14 2021 at 11:42, Paolo Bonzini wrote:
    > On 12/14/21 03:50, Thomas Gleixner wrote:
    >> The only remaining issue is the KVM XSTATE save/restore size checking which
    >> probably requires some FPU core assistance. But that requires some more
    >> thoughts vs. the IOCTL interface extension and once that is settled it
    >> needs to be solved in one go. But that's an orthogonal issue to the above.
    >
    > That's not a big deal because KVM uses the uncompacted format. So
    > KVM_CHECK_EXTENSION and KVM_GET_XSAVE can just use CPUID to retrieve the
    > size and uncompacted offset of the largest bit that is set in
    > kvm_supported_xcr0, while KVM_SET_XSAVE can do the same with the largest
    > bit that is set in the xstate_bv.

    For simplicity you can just get that information from guest_fpu. See
    below.

    Thanks,

    tglx
    ---
    --- a/arch/x86/include/asm/fpu/types.h
    +++ b/arch/x86/include/asm/fpu/types.h
    @@ -518,6 +518,11 @@ struct fpu_guest {
    u64 perm;

    /*
    + * @uabi_size: Size required for save/restore
    + */
    + unsigned int uabi_size;
    +
    + /*
    * @fpstate: Pointer to the allocated guest fpstate
    */
    struct fpstate *fpstate;
    --- a/arch/x86/kernel/fpu/core.c
    +++ b/arch/x86/kernel/fpu/core.c
    @@ -240,6 +240,7 @@ bool fpu_alloc_guest_fpstate(struct fpu_
    gfpu->fpstate = fpstate;
    gfpu->xfeatures = fpu_user_cfg.default_features;
    gfpu->perm = fpu_user_cfg.default_features;
    + gfpu->uabi_size = fpu_user_cfg.default_size;
    fpu_init_guest_permissions(gfpu);

    return true;
    --- a/arch/x86/kernel/fpu/xstate.c
    +++ b/arch/x86/kernel/fpu/xstate.c
    @@ -1545,6 +1545,7 @@ static int fpstate_realloc(u64 xfeatures
    newfps->is_confidential = curfps->is_confidential;
    newfps->in_use = curfps->in_use;
    guest_fpu->xfeatures |= xfeatures;
    + guest_fpu->uabi_size = usize;
    }

    fpregs_lock();
    \
     
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