On Sun, Jul 12, 2020 at 6:16 PM Yv Lin <yvl...@gmail.com> wrote: > > Here are some summaries that I learned from what you told. > 1) If a device is passed through to guestOS via vfio, and there is no > IOMMU present in guestOS. all memory regions within the device address > space will be pinned down. if IOMMU is presented in guestOS, qemu could > only pin and map the needed pages (specified by dma_map_page() called in > guestOS device driver), but as vIOMMU is emulated, the performance is not > good. > 2) Even if a pcie device can support ATS/PRI capability and it's passed > through to guestOS, the above statement is still true, the IO page fault > and demanding page won't be utilized anyway. >
Correct, also note that a vIOMMU is never enabled during early boot on x86/64, therefore all guest memory will be pinned initially. Also a vIOMMU introduces locked memory accounting issues as each device address space makes use of a separate VFIO container, which does accounting separately. And finally, ATS implies that we honor devices making use of "pre-translated" DMA, which implies a degree of trust that the user/device cannot make use of this as a vector to exploit the host. Thanks, Alex
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