On Thu, Nov 21, 2013 at 10:25:52AM +0200, Gleb Natapov wrote:
> On Thu, Nov 21, 2013 at 10:25:34AM +0200, Michael S. Tsirkin wrote:
> > On Thu, Nov 21, 2013 at 10:21:33AM +0200, Michael S. Tsirkin wrote:
> > > On Thu, Nov 21, 2013 at 10:18:34AM +0200, Michael S. Tsirkin wrote:
> > > > On Thu, Nov 21, 2013 at 10:16:27AM +0200, Michael S. Tsirkin wrote:
> > > > > On Thu, Nov 21, 2013 at 10:01:27AM +0200, Gleb Natapov wrote:
> > > > > > 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
> > > > >
> > > > > OK this one I'm familiar with, it describes how PAT
> > > > > interacts with MTRR. But how does this interact with EPT?
> > > > > do you remember where's that described?
> > > >
> > > > Found it 28.2.4 EPT and Memory Typing
> > > > sorry about the noise.
> > >
> > >
> > > OK and that's explicit:
> > > If CR0.CD = 0, the effective memory type depends upon the value of bit
> > > 6 of the last EPT paging-structure entry:
> > > — If the value is 0, the effective memory type is the combination of the
> > > EPT memory type and the PAT memory type specified in Table 11-7 in Section
> > > 11.5.2.2, using the EPT memory type in place of the MTRR memory type.
> > >
> > > — If the value is 1, the memory type used for the access is the EPT
> > > memory
> > > type. The PAT memory type is ignored.
> > >
> > > If CR0.CD = 1, the effective memory type is UC.
> > >
> > > So it's simple. EPT replaces guest's MTRR.
> >
> >
> > And that in turn means that since we set UC in EPT,
> > VCPU will always work as if it's UC except for
> > guests using WC - WC takes precedence.
> >
> That's what I wrote below, no? :)
Yes I just added that this applies to other memory types
like WT (that probably nobody uses?).
> >
> > Thanks!
> > > >
> > > > > > > > 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.
>
> --
> Gleb.
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