Re: [edk2-devel] [PATCH V1 1/1] UefiCpuPkg/ResetVector: Cache Disable should not be set by default in CR0

["Gerd Hoffmann" <kraxel@redhat.com>]

  Hi,

> So, yesterday I read your status on virt-staff, and I found an entry in
> it that resembled this upstream thread pretty closely. However, your
> status was the *only* mention of "mdev" specifically, and so I wasn't
> sure if *mdev* meant the same thing as the more generic upstream
> expression "pci device assignment" (see it above in the context).
> 
> Furthermore, I saw kvm_arch_has_noncoherent_dma() in my linux commit
> 879ae1880449, which superficially resembled device assignment, but... I
> dismissed it. In the end, I only managed (and even that, only
> reluctantly) the above pointers... Thanks for tracking it down!
> 
> But then, next question: why has this problem *not* been reported
> repeatedly? There's a whole bunch of users (gamers) that run Windows
> guests with device (GPU) assignment. I'm sure they'd absolutely complain
> about very slow OVMF boot (like they actually have, in the past, about
> similar LZMA slowdowns due to improper caching setup).

static u8 vmx_get_mt_mask(struct kvm_vcpu *vcpu, gfn_t gfn, bool is_mmio)
{
[ ... ]
         * When there is no need to deal with noncoherent DMA (e.g., no VT-d
         * or VT-d has snoop control), guest CD/MTRR/PAT are all ignored.  The
         * EPT memory type is set to WB.  The effective memory type is forced
         * WB.
         *
         * Otherwise, we trust guest.  Guest CD/MTRR/PAT are all honored.  The
         * EPT memory type is used to emulate guest CD/MTRR.
[ ... ]

> Something must be special about Min's assigned device.

Yep.  I think the magic word is "snoop control".  When pci-assigning a
*real* pci device VT-d (aka iommu) handles cache control that way.  When
assigning a mdev device this is not the case.

mdev is a virtual pci device emulated by the kernel.  This can be purely
virtual (see samples/vfio-mdev/mtty.c in the linux kernel, which can be
used to reproduce this).  More typical is hardware-assisted device
partitioning, used for some intel and nvidia gpus.  Roughly comparable
with SR/IOV, but not implemented completely in hardware, the kernel has
some device-specific support code instead.

take care,
  Gerd



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