Thanks David for all the analysis. > On 11 Jul 2024, at 4:53 PM, David Woodhouse <dw...@infradead.org> wrote: > > On Thu, 2024-07-11 at 08:26 +0100, David Woodhouse wrote: >> >> I used identical command lines on both, and on each host I got the same >> result with all of '-cpu host', '-cpu EPYC' and -cpu Skylake-Server'. >> It's the *host* that makes the difference, not the CPUID presented to >> the guest. > > Actually... it turns out QEMU isn't really advertising the CPUID we ask > it to. Leaf zero still does say 'AuthenticAMD' vs. GenuineIntel' > according to the *host* it's running on, regardless of the -cpu option. > that undermines advertised cpu model.
> And it is indeed *just* that which seems to trigger the Windows bug, > setting IRQ2 to point somewhere bogus: > > vtd_ir_remap_msi_req addr 0xfee00004 data 0x0 sid 0xffff do_fault 0 > vtd_ir_remap_msi (addr 0xfee00004, data 0x0) -> (addr 0xfee00004, data 0x0) > kvm_irqchip_update_msi_route Updating MSI route virq=2 > > While in the happy case it does use a remappable format MSI message. > Not direct to vector 209 on CPU0 as I said before; I think that's IRTE > entry #209 which maps to vector 209 on CPU1. > > vtd_ir_remap_msi_req addr 0xfee00010 data 0xd1 sid 0xffff do_fault 0 > vtd_ir_irte_get index 0 low 0x0 high 0x100d10005 > vtd_ir_remap index 0 trigger 0 vector 209 deliver 0 dest 0x1 mode 1 > vtd_ir_remap_type IOAPIC > vtd_ir_remap_msi (addr 0xfee00010, data 0xd1) -> (addr 0xfee01004, data > 0x40d1) > > So it looks like Windows doesn't actually cope with Intel IRQ remapping > when it sees and AMD CPU, which is suboptimal. > Makes sense. > So to support >255 vCPUs on AMD without having to also do *DMA* > translation, either we need to come up with a trick like the "no > supported address widths" we use for dma-translation=off on Intel, or > we see if we can persuade Windows to use the 15-bit MSI support. > > > Looking at the Linux guest support, it seems to look just at the HyperV > CPUID leaves 0x40000081 and 0x40000082. QEMU knows of those only for > SYNDBG; Sandesh do you want to try setting the > HYPERV_VS_PROPERTIES_EAX_EXTENDED_IOAPIC_RTE bit that Linux looks for, > and see how that affects Windows guests (with no emulated IOMMU)? > Sure I would try that, I would need some reading time however. Regards, Sandesh
