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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