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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; > > > > 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 > > ------------------------------------------------------ I re-run all the tests and collected an averaged data (10 times each) as below (previous data was just a single-run score): T=Test execution time of a selftest in which guest writes to a GPA for 0x1000000UL times with WRITE_ONCE KVM memtype | Sapphire Rapids XCC | Coffee Lake-S -------------|---------------------|---------------- WB+IPAT | T=0.1511s | T=0.1661s -------------|---------------------|---------------- WC+IPAT | T=0.1411s | T=0.1656s -------------|---------------------|---------------- WT+IPAT | T=3.7527s | T=0.6156s -------------|---------------------|---------------- WP+IPAT | T=4.4663s | T=0.6203s -------------|---------------------|---------------- UC+IPAT | T=3.4632s | T=0.5868s T=Test execution time of a selftest in which guest writes to a GPA for 0x1000000UL times with movdir64b. (Coffee Lake-S has no feature movdir64). KVM memtype | Sapphire Rapids XCC | Coffee Lake-S -------------|---------------------|---------------- WB+IPAT | T=2.6142s | / -------------|---------------------|---------------- WC+IPAT | T=2.8919s | / -------------|---------------------|---------------- WT+IPAT | T=3.0966s | / -------------|---------------------|---------------- WP+IPAT | T=2.4933s | / -------------|---------------------|---------------- UC+IPAT | T=3.4606s | /