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

[David Vrabel <david.vrabel@citrix.com>]

On 17/01/13 18:44, Boris Ostrovsky wrote:
> 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@amd.com>
> 
> 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

This register in the comment doesn't seem to agree with the one used below.

> +	 */
> +	if (c->x86 == 0x10) {
> +		rdmsr_safe(0xc001102a, value);
> +		value &= ~(1ULL << 24);
> +		wrmsr_safe(0xc001102a, value);
> +	}

David