Re: [Xen-devel] [V6 4/4] libxc: expose xsaves/xgetbv1/xsavec to hvm guest

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 12/10/15 07:07, Shuai Ruan wrote:
> This patch exposes xsaves/xgetbv1/xsavec to hvm guest.
> The reserved bits of eax/ebx/ecx/edx must be cleaned up
> when call cpuid(0dh) with leaf 1 or 2..63.
>
> According to the spec the following bits must be reserved:
> For leaf 1, bits 03-04/08-31 of ecx is reserved. Edx is reserved.
> For leaf 2...63, bits 01-31 of ecx is reserved. Edx is reserved.
>
> Acked-by: Ian Campbell <ian.campbell@xxxxxxxxxx>
> Signed-off-by: Shuai Ruan <shuai.ruan@xxxxxxxxxxxxxxx>

This patch needs rebasing over c/s bf87f3a "tools/libxc: Improve
efficiency of xc_cpuid_apply_policy()"

> ---
>  tools/libxc/xc_cpuid_x86.c | 16 ++++++++++++----
>  1 file changed, 12 insertions(+), 4 deletions(-)
>
> diff --git a/tools/libxc/xc_cpuid_x86.c b/tools/libxc/xc_cpuid_x86.c
> index e146a3e..b0d6ecc 100644
> --- a/tools/libxc/xc_cpuid_x86.c
> +++ b/tools/libxc/xc_cpuid_x86.c
> @@ -210,6 +210,10 @@ static void intel_xc_cpuid_policy(
>  }
>  
>  #define XSAVEOPT        (1 << 0)
> +#define XSAVEC          (1 << 1)
> +#define XGETBV1         (1 << 2)
> +#define XSAVES          (1 << 3)
> +#define XSS_SUPPORT     (1 << 0)
>  /* Configure extended state enumeration leaves (0x0000000D for xsave) */
>  static void xc_cpuid_config_xsave(
>      xc_interface *xch, domid_t domid, uint64_t xfeature_mask,
> @@ -246,8 +250,9 @@ static void xc_cpuid_config_xsave(
>          regs[1] = 512 + 64; /* FP/SSE + XSAVE.HEADER */
>          break;
>      case 1: /* leaf 1 */
> -        regs[0] &= XSAVEOPT;
> -        regs[1] = regs[2] = regs[3] = 0;
> +        regs[0] &= (XSAVEOPT | XSAVEC | XGETBV1 | XSAVES);

The correct mask here depends on !info->hvm, to hide XSAVES from PV guests.

> +        regs[2] &= xfeature_mask;
> +        regs[3] = 0;
>          break;
>      case 2 ... 63: /* sub-leaves */
>          if ( !(xfeature_mask & (1ULL << input[1])) )
> @@ -255,8 +260,11 @@ static void xc_cpuid_config_xsave(
>              regs[0] = regs[1] = regs[2] = regs[3] = 0;
>              break;
>          }
> -        /* Don't touch EAX, EBX. Also cleanup ECX and EDX */
> -        regs[2] = regs[3] = 0;
> +        /* Don't touch EAX, EBX and cleanup EDX.
> +         * Bit 0 of ECX represent whether sub-leave is supported in IA32_XSS
> +         * msr or supported in XCR0.*/
> +        regs[2] &= XSS_SUPPORT;

No XSS features are currently supported, even in HVM guests.  This
should be unilaterally zero at the moment.

~Andrew

> +        regs[3] = 0;
>          break;
>      }
>  }


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