Re: [PATCH 14/15] KVM: MTRR: do not map huage page for non-consistent range

[Xiao Guangrong]

On 06/04/2015 04:23 PM, Xiao Guangrong wrote:
> On 06/03/2015 03:55 PM, Paolo Bonzini wrote:
> > On 03/06/2015 04:56, Xiao Guangrong wrote:
> > > On 06/01/2015 05:36 PM, Paolo Bonzini wrote:
> > > > On 30/05/2015 12:59, Xiao Guangrong wrote:
> > > > > Currently guest MTRR is completely prohibited if cache snoop is
> > > > > supported on
> > > > > IOMMU (!noncoherent_dma) and host does the emulation based on the
> > > > > knowledge
> > > > > from host side, however, host side is not the good point to know
> > > > > what the purpose of guest is. A good example is that pass-throughed VGA
> > > > > frame buffer is not always UC as host expected
> > > >
> > > > Can you explain how?  The original idea was that such a framebuffer
> > > > would be kvm_is_reserved_pfn and thus be unconditionally UC.
> > >
> > > Yes, frame-buffer is always UC in current code, however, UC for
> > > frame-buffer causes bad performance.
> >
> > Understood now, thanks.
> >
> > > So that guest will configure the range to MTRR, this patchset follows
> > > guest MTRR and cooperates with guest PAT (ept.VMX_EPT_IPAT_BIT = 0) to
> > > emulate guest cache type as guest expects.
> >
> > Unlike e.g. CR0.CD=1, UC memory does not snoop the cache to preserve
> > coherency.  AMD, has special logic to do this, for example:
> >
> > - if guest PAT says "UC" and host MTRR says "WB", the processor will not
> > cache the memory but will snoop the cache as if CR0.CD=1
> >
> > - if guest PAT says "WC" and host (nested page table) PAT says "WB" and
> > host MTRR says "WB", the processor will still do write combining but
> > also snoop the cache as if CR0.CD=1
> >
> > I am worried that the lack of this feature could cause problems if
> > guests map QEMU's VGA framebuffer as uncached.  We have this problem on
> > ARM, so it's not 100% theoretical.
>
> CR0.CD is always 0 in both host and guest, i guess it's why we cleared
> CR0.CD and CR0.NW in vmx_set_cr0().

It reminds me that we should check guest CR0.CD before check guest MTRR
and disable guest PAT if guest CR0.CD = 1.

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/