On Thu, Nov 21, 2013 at 09:18:55AM +0200, Gleb Natapov wrote:
> On Wed, Nov 20, 2013 at 09:58:15PM +0200, Michael S. Tsirkin wrote:
> > I see this in kvm:
> >
> > static u64 vmx_get_mt_mask(struct kvm_vcpu *vcpu, gfn_t gfn, bool
> > is_mmio)
> > {
> > u64 ret;
> >
> > /* For VT-d and EPT combination
> > * 1. MMIO: always map as UC
> > * 2. EPT with VT-d:
> > * a. VT-d without snooping control feature: can't guarantee
> > * the
> > * result, try to trust guest.
> > * b. VT-d with snooping control feature: snooping control
> > * feature of
> > * VT-d engine can guarantee the cache correctness. Just
> > * set it
> > * to WB to keep consistent with host. So the same as item
> > * 3.
> > * 3. EPT without VT-d: always map as WB and set IPAT=1 to keep
> > * consistent with host MTRR
> > */
> > if (is_mmio)
> > ret = MTRR_TYPE_UNCACHABLE << VMX_EPT_MT_EPTE_SHIFT;
> >
> > ...
> > }
> >
> >
> > does this mean that even if guest maps BAR for an assigned device
> > as write combined (or configures such using an MTRR),
> > host will override this and use uncacheable in practice?
> >
> No, it does not mean that. I already answered this once (my previous
> answer included below): effective memory type is a combination of MTRR
> (EPT MT bits in case of a guest) and PAT bits. See section 11.5.2.2
> in SDM
Can you quote chapter name please?
My SDM has
11.5.2.2 Denormal-Operand Exception (#D)
> on how effective memory type is calculated.
>
> Since MTRR UC + PAT WC = WC, if guest maps MMIO as WC in a page table
> (that what ioremap_wc does), everything works as it should. If guest maps
> MMIO as WB (ioremap_cache) and MTRR says MMIO is UC (like any MMIO will
> be by default) combined memory type will be UC, so also fine. If guest
> maps MMIO range as WB and fixes mtrr for this region to be WB then memory
> type will be incorrect in a guest,
Meaning MTRR in guest is ignored in this case?
> but I found only one place that does
> it in Linux: drivers/video/vesafb.c. All other uses of ioremap_cache
> either remap RAM or used to get whatever memory type configured in MTRR.
>
> --
> Gleb.
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