Re: [V7 1/4] x86/xsaves: add basic definitions/helpers to support xsaves

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 20/10/15 09:21, Shuai Ruan wrote:
> This patch add basic definitions/helpers which will be used in
> later patches.
>
> Signed-off-by: Shuai Ruan <shuai.ruan@linux.intel.com>
> Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
> ---
>  xen/arch/x86/xstate.c           | 19 +++++++++++++++++++
>  xen/include/asm-x86/hvm/vcpu.h  |  1 +
>  xen/include/asm-x86/msr-index.h |  2 ++
>  xen/include/asm-x86/xstate.h    | 12 ++++++++++--
>  4 files changed, 32 insertions(+), 2 deletions(-)
>
> diff --git a/xen/arch/x86/xstate.c b/xen/arch/x86/xstate.c
> index 9ddff90..4230066 100644
> --- a/xen/arch/x86/xstate.c
> +++ b/xen/arch/x86/xstate.c
> @@ -24,6 +24,9 @@ static u32 __read_mostly xsave_cntxt_size;
>  /* A 64-bit bitmask of the XSAVE/XRSTOR features supported by processor. */
>  u64 __read_mostly xfeature_mask;
>  
> +/* Cached xss for fast read */
> +static DEFINE_PER_CPU(uint64_t, xss);
> +
>  /* Cached xcr0 for fast read */
>  static DEFINE_PER_CPU(uint64_t, xcr0);
>  
> @@ -60,6 +63,22 @@ uint64_t get_xcr0(void)
>      return this_cpu(xcr0);
>  }
>  
> +void set_msr_xss(u64 xss)
> +{
> +    u64 *this_xss = &this_cpu(xss);
> +
> +    if ( *this_xss != xss )
> +    {
> +        wrmsrl(MSR_IA32_XSS, xss);
> +        this_cpu(xss) = xss;

*this_xss = xss;

Using this_cpu() multiple times cannot be optimised by compiler (because
of the underlying definition), which is why the optimisation is
performed manually by pulling the value out as a pointer.

~Andrew