Re: [Xen-devel] [PATCH] AMD: Enable WC+ memory type on family 10 processors

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 17/01/13 18:44, Boris Ostrovsky wrote:
> # HG changeset patch
> # User Boris Ostrovsky <boris.ostrovsky@xxxxxxx>
> # Date 1358429914 18000
> # Node ID 1963e398ba0f5dd61818d92400c7b07a32f6da4b
> # Parent  b6195e277da52129d5d79f29ccac0e7290643e37
> AMD: Enable WC+ memory type on family 10 processors
>
> In some cases BIOS may not enable WC+ memory type on family 10 processors,
> instead converting what would be WC+ memory to CD type. On guests using
> nested pages this could result in performance degradation. This patch
> enables WC+.
>
> Signed-off-by: Boris Ostrovsky <boris.ostrovsky@xxxxxxx>
>
> diff -r b6195e277da5 -r 1963e398ba0f xen/arch/x86/cpu/amd.c
> --- a/xen/arch/x86/cpu/amd.c  Wed Jan 16 14:15:44 2013 +0000
> +++ b/xen/arch/x86/cpu/amd.c  Thu Jan 17 08:38:34 2013 -0500
> @@ -495,6 +495,18 @@ static void __devinit init_amd(struct cp
>               set_bit(X86_FEATURE_ARAT, c->x86_capability);
>  
>       /*
> +      * On family 10h BIOS may not have properly enabled WC+ support, causing
> +      * it to be converted to CD memtype. This may result in performance
> +      * degradation for certain nested-paging guests. Prevent this conversion
> +      * by enabling WC+ support through MSRC001_1024A[24] = 0
> +      */
> +     if (c->x86 == 0x10) {
> +             rdmsr_safe(0xc001102a, value);
> +             value &= ~(1ULL << 24);
> +             wrmsr_safe(0xc001102a, value);
> +     }

Purely on style rather than content:

If we know the family type and can be certain that the MSRs are good,
then you should not be using the _safe variants.

If we don't know for certain that the MSRs are safe to use, then errors
should be caught and at the very least use warn the user.

Secondly, can we name the MSRs rather than using magic values.

~Andrew

> +
> +     /*
>        * Prior to Family 0x14, perf counters are not reset during warm reboot.
>        * We have to reset them manually.
>        */
>
>
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