Re: [CFT PATCH 2/2] KVM: x86: support XSAVES usage in the host

[Andy Lutomirski <luto@amacapital.net>]

On 11/21/2014 10:31 AM, Paolo Bonzini wrote:
> Userspace is expecting non-compacted format for KVM_GET_XSAVE, but
> struct xsave_struct might be using the compacted format.  Convert
> in order to preserve userspace ABI.
> 
> Fixes: f31a9f7c71691569359fa7fb8b0acaa44bce0324
> Cc: Fenghua Yu <fenghua.yu@intel.com>
> Cc: stable@vger.kernel.org
> Cc: Nadav Amit <namit@cs.technion.ac.il>
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> ---
>  arch/x86/kvm/x86.c | 48 +++++++++++++++++++++++++++++++++++++++++++-----
>  1 file changed, 43 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 5337039427c8..7e8a20e5615a 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -3131,15 +3131,53 @@ static int kvm_vcpu_ioctl_x86_set_debugregs(struct kvm_vcpu *vcpu,
>  	return 0;
>  }
>  
> +#define XSTATE_COMPACTION_ENABLED (1ULL << 63)
> +
> +static void fill_xsave(u8 *dest, struct kvm_vcpu *vcpu)
> +{
> +	struct xsave_struct *xsave = &vcpu->arch.guest_fpu.state->xsave;
> +	u64 xstate_bv = vcpu->arch.guest_supported_xcr0 | XSTATE_FPSSE;
> +	u64 valid;
> +
> +	/*
> +	 * Copy legacy XSAVE area, to avoid complications with CPUID
> +	 * leaves 0 and 1 in the loop below.
> +	 */
> +	memcpy(dest, xsave, XSAVE_HDR_OFFSET);
> +
> +	/* Set XSTATE_BV */
> +	*(u64 *)(dest + XSAVE_HDR_OFFSET) = xstate_bv;
> +
> +	/*
> +	 * Copy each region from the possibly compacted offset to the
> +	 * non-compacted offset.
> +	 */
> +	valid = xstate_bv & ~XSTATE_FPSSE;
> +	if (xsave->xsave_hdr.xcomp_bv & XSTATE_COMPACTION_ENABLED)
> +		valid &= xsave->xsave_hdr.xcomp_bv;
> +
> +	while (valid) {
> +		u64 feature = valid & -valid;
> +		int index = fls64(feature) - 1;
> +		void *src = get_xsave_addr(xsave, feature);
> +
> +	        if (src) {
> +			u32 size, offset, ecx, edx;
> +			cpuid_count(XSTATE_CPUID, index,
> +				    &size, &offset, &ecx, &edx);
> +	                memcpy(dest + offset, src, size);

Is this really the best way to do this?  cpuid is serializing, so this
is possibly *very* slow.

--Andy