Re: guest assigned device MMIO maps with WC: does this work correctly?

["Michael S. Tsirkin" <mst@redhat.com>]

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.