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.
WIP to find out why effective UC in fb_map range will make gdm to restart
endlessly.
