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

[Yan Zhao <yan.y.zhao@intel.com>]

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@google.com> 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.
> 
> 
> WIP to find out why effective UC in fb_map range will make gdm to restart
> endlessly.
Not sure whether it's simply because UC is too slow.

T=Test execution time of a selftest in which guest writes to a GPA for
  0x1000000UL times

              | Sapphire Rapids XCC  | Coffee Lake-S
--------------|----------------------|-----------------
KVM UC+IPAT   |    T=0m4.530s        |  T=0m0.622s
--------------|----------------------|-----------------
KVM WC+IPAT   |    T=0m0.149s        |  T=0m0.176s
--------------|----------------------|-----------------
KVM WB+IPAT   |    T=0m0.148s        |  T=0m0.148s
------------------------------------------------------