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

---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;
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