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.
> > 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
Woah. Have you tried testing MOVDIR64 and/or WT? E.g. to see if the problem is
with UC specifically, or if it occurs with any accesses that immediately write
through to main memory.
> --------------|----------------------|-----------------
> KVM WC+IPAT | T=0m0.149s | T=0m0.176s
> --------------|----------------------|-----------------
> KVM WB+IPAT | T=0m0.148s | T=0m0.148s
> ------------------------------------------------------
