Re: [PATCH 5/5] KVM: VMX: Always honor guest PAT on CPUs that support self-snoop

[Sean Christopherson]

On Mon, Sep 09, 2024, Paolo Bonzini wrote:
> On 9/9/24 07:30, Yan Zhao wrote:
> > On Thu, Sep 05, 2024 at 05:43:17PM +0800, Yan Zhao wrote:
> > > On Wed, Sep 04, 2024 at 05:41:06PM -0700, Sean Christopherson wrote:
> > > > On Wed, Sep 04, 2024, Yan Zhao wrote:
> > > > > On Wed, Sep 04, 2024 at 10:28:02AM +0800, Yan Zhao wrote:
> > > > > > On Tue, Sep 03, 2024 at 06:20:27PM +0200, Vitaly Kuznetsov wrote:
> > > > > > > Sean Christopherson <seanjc@xxxxxxxxxx> writes:
> > > > > > > 
> > > > > > > > On Mon, Sep 02, 2024, Vitaly Kuznetsov wrote:
> > > > > > > > > FWIW, I use QEMU-9.0 from the same C10S (qemu-kvm-9.0.0-7.el10.x86_64)
> > > > > > > > > but I don't think it matters in this case. My CPU is "Intel(R) Xeon(R)
> > > > > > > > > Silver 4410Y".
> > > > > > > > 
> > > > > > > > Has this been reproduced on any other hardware besides SPR?  I.e. did we stumble
> > > > > > > > on another hardware issue?
> > > > > > > 
> > > > > > > Very possible, as according to Yan Zhao this doesn't reproduce on at
> > > > > > > least "Coffee Lake-S". Let me try to grab some random hardware around
> > > > > > > and I'll be back with my observations.
> > > > > > 
> > > > > > Update some new findings from my side:
> > > > > > 
> > > > > > BAR 0 of bochs VGA (fb_map) is used for frame buffer, covering phys range
> > > > > > from 0xfd000000 to 0xfe000000.
> > > > > > 
> > > > > > On "Sapphire Rapids XCC":
> > > > > > 
> > > > > > 1. If KVM forces this fb_map range to be WC+IPAT, installer/gdm can launch
> > > > > >     correctly.
> > > > > >     i.e.
> > > > > >     if (gfn >= 0xfd000 && gfn < 0xfe000) {
> > > > > >     	return (MTRR_TYPE_WRCOMB << VMX_EPT_MT_EPTE_SHIFT) | VMX_EPT_IPAT_BIT;
> > > > > >     }
> > > > > >     return MTRR_TYPE_WRBACK << VMX_EPT_MT_EPTE_SHIFT;
> > > > > > 
> > > > > > 2. If KVM forces this fb_map range to be UC+IPAT, installer failes to show / gdm
> > > > > >     restarts endlessly. (though on Coffee Lake-S, installer/gdm can launch
> > > > > >     correctly in this case).
> > > > > > 
> > > > > > 3. On starting GDM, ttm_kmap_iter_linear_io_init() in guest is called to set
> > > > > >     this fb_map range as WC, with
> > > > > >     iosys_map_set_vaddr_iomem(&iter_io->dmap, ioremap_wc(mem->bus.offset, mem->size));
> > > > > > 
> > > > > >     However, during bochs_pci_probe()-->bochs_load()-->bochs_hw_init(), pfns for
> > > > > >     this fb_map has been reserved as uc- by ioremap().
> > > > > >     Then, the ioremap_wc() during starting GDM will only map guest PAT with UC-.
> > > > > > 
> > > > > >     So, with KVM setting WB (no IPAT) to this fb_map range, the effective
> > > > > >     memory type is UC- and installer/gdm restarts endlessly.
> > > > > > 
> > > > > > 4. If KVM sets WB (no IPAT) to this fb_map range, and changes guest bochs driver
> > > > > >     to call ioremap_wc() instead in bochs_hw_init(), gdm can launch correctly.
> > > > > >     (didn't verify the installer's case as I can't update the driver in that case).
> > > > > > 
> > > > > >     The reason is that the ioremap_wc() called during starting GDM will no longer
> > > > > >     meet conflict and can map guest PAT as WC.
> > > > 
> > > > Huh.  The upside of this is that it sounds like there's nothing broken with WC
> > > > or self-snoop.
> > > Considering a different perspective, the fb_map range is used as frame buffer
> > > (vram), with the guest writing to this range and the host reading from it.
> > > If the issue were related to self-snooping, we would expect the VNC window to
> > > display distorted data. However, the observed behavior is that the GDM window
> > > shows up correctly for a sec and restarts over and over.
> > > 
> > > So, do you think we can simply fix this issue by calling ioremap_wc() for the
> > > frame buffer/vram range in bochs driver, as is commonly done in other gpu
> > > drivers?
> > > 
> > > --- a/drivers/gpu/drm/tiny/bochs.c
> > > +++ b/drivers/gpu/drm/tiny/bochs.c
> > > @@ -261,7 +261,9 @@ static int bochs_hw_init(struct drm_device *dev)
> > >          if (pci_request_region(pdev, 0, "bochs-drm") != 0)
> > >                  DRM_WARN("Cannot request framebuffer, boot fb still active?\n");
> > > 
> > > -       bochs->fb_map = ioremap(addr, size);
> > > +       bochs->fb_map = ioremap_wc(addr, size);
> > >          if (bochs->fb_map == NULL) {
> > >                  DRM_ERROR("Cannot map framebuffer\n");
> > >                  return -ENOMEM;
> 
> While this is a fix for future kernels, it doesn't change the result for VMs
> already in existence.

I would prefer to bottom out on exactly whether or not the SPR/CLX behavior is
working as intended.  Maybe the ~8x slowdown is just a side effect of any Intel
multi-socket/node system, but I think we should get confirmation (inasmuch as
possible) that that is indeed the case.  E.g. if this is actually a bug in CLX+,
then the actions we need to take are different.

> I don't think there's an alternative to putting this behind a quirk.

This gets a bit weird, which is why I want to bottom out on whether or not CLX
and SPR are working as intended.  If non-coherent DMA is attached to the VM, then
even before this patch KVM would honor guest PAT.  I agree that we don't want to
break existing setups, but if CLX+SPR are working as intended, then this is
inarguably a bochs driver bug, and I would prefer to have the quirk explicitly
reference bochs-compatible devices, e.g. in the name and documentation, so that
userspace can disable the quirk by default and only leave it enabled if a bochs
device is being exposed to the guest.