On Thu, Nov 21, 2013 at 10:02:07AM +0200, Michael S. Tsirkin wrote:
> 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)
>
Either your or mine is out of date:
Selecting Memory Types for Pentium III and More Recent Processor Families
> > 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?
Yes.
>
> > 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.
--
Gleb.
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