Re: [Xen-devel] [PATCH v2 1/3] xen/pvh: enable and set default MTRR type

[Juergen Gross <jgross@suse.com>]

On 09/05/18 17:11, Roger Pau Monné wrote:
> On Wed, May 09, 2018 at 12:30:16PM +0100, Roger Pau Monné wrote:
>> On Wed, May 09, 2018 at 11:56:40AM +0100, Andrew Cooper wrote:
>>> On 09/05/18 11:21, Roger Pau Monne wrote:
>>> I'm not sure that setting the default MTRR type is going to be a
>>> clever idea in hindsight when we come to doing PCI Passthrough support.
>>
>> Setting the default type to WB is also set by hvmloader, it's just
>> that hvmloader also sets some of the fixed and variable ranges to UC
>> in order to cover the iomem areas.
>>
>> The expectations when doing pci-passthrough is that the guest will
>> always use paging and PAT in order to set the appropriate cache
>> attributes, or else the guest itself will have to program the UC MTRR
>> ranges, I admit that's not very nice however.
>>
>> What about enabling the default MTRR type and setting it to WB in the
>> toolstack for PVH? IMO doing it Xen itself would be wrong.
> 
> I have the following patch to set the default MTRR type, but I think
> if we go down this road then we will also have to set UC MTRRs for
> MMIO areas, which again seems fine to me.

I like this route much better.


Juergen

> 
> ---8<---
> diff --git a/tools/libxc/xc_dom_x86.c b/tools/libxc/xc_dom_x86.c
> index e33a28847d..3cb1a1720f 100644
> --- a/tools/libxc/xc_dom_x86.c
> +++ b/tools/libxc/xc_dom_x86.c
> @@ -938,6 +938,8 @@ static int vcpu_hvm(struct xc_dom_image *dom)
>          HVM_SAVE_TYPE(HEADER) header;
>          struct hvm_save_descriptor cpu_d;
>          HVM_SAVE_TYPE(CPU) cpu;
> +        struct hvm_save_descriptor mtrr_d;
> +        HVM_SAVE_TYPE(MTRR) mtrr;
>          struct hvm_save_descriptor end_d;
>          HVM_SAVE_TYPE(END) end;
>      } bsp_ctx;
> @@ -1014,6 +1016,15 @@ static int vcpu_hvm(struct xc_dom_image *dom)
>      if ( dom->start_info_seg.pfn )
>          bsp_ctx.cpu.rbx = dom->start_info_seg.pfn << PAGE_SHIFT;
>  
> +    /* Set the MTRR. */
> +    bsp_ctx.mtrr_d.typecode = HVM_SAVE_CODE(MTRR);
> +    bsp_ctx.mtrr_d.instance = 0;
> +    bsp_ctx.mtrr_d.length = HVM_SAVE_LENGTH(MTRR);
> +    /* XXX: maybe this should be a firmware option instead? */
> +    if ( !dom->device_model )
> +        /* Enable MTRR (bit 11) and set the default type to WB (6). */
> +        bsp_ctx.mtrr.msr_mtrr_def_type = (1u << 11) | 6;
> +
>      /* Set the end descriptor. */
>      bsp_ctx.end_d.typecode = HVM_SAVE_CODE(END);
>      bsp_ctx.end_d.instance = 0;
> 
>