On Mon, Apr 04, 2022, Quentin Perret wrote: > On Friday 01 Apr 2022 at 12:56:50 (-0700), Andy Lutomirski wrote: > FWIW, there are a couple of reasons why I'd like to have in-place > conversions: > > - one goal of pKVM is to migrate some things away from the Arm > Trustzone environment (e.g. DRM and the likes) and into protected VMs > instead. This will give Linux a fighting chance to defend itself > against these things -- they currently have access to _all_ memory. > And transitioning pages between Linux and Trustzone (donations and > shares) is fast and non-destructive, so we really do not want pKVM to > regress by requiring the hypervisor to memcpy things; Is there actually a _need_ for the conversion to be non-destructive? E.g. I assume the "trusted" side of things will need to be reworked to run as a pKVM guest, at which point reworking its logic to understand that conversions are destructive and slow-ish doesn't seem too onerous. > - it can be very useful for protected VMs to do shared=>private > conversions. Think of a VM receiving some data from the host in a > shared buffer, and then it wants to operate on that buffer without > risking to leak confidential informations in a transient state. In > that case the most logical thing to do is to convert the buffer back > to private, do whatever needs to be done on that buffer (decrypting a > frame, ...), and then share it back with the host to consume it; If performance is a motivation, why would the guest want to do two conversions instead of just doing internal memcpy() to/from a private page? I would be quite surprised if multiple exits and TLB shootdowns is actually faster, especially at any kind of scale where zapping stage-2 PTEs will cause lock contention and IPIs. > - similar to the previous point, a protected VM might want to > temporarily turn a buffer private to avoid ToCToU issues; Again, bounce buffer the page in the guest. > - once we're able to do device assignment to protected VMs, this might > allow DMA-ing to a private buffer, and make it shared later w/o > bouncing. Exposing a private buffer to a device doesn't requring in-place conversion. The proper way to handle this would be to teach e.g. VFIO to retrieve the PFN from the backing store. I don't understand the use case for sharing a DMA'd page at a later time; with whom would the guest share the page? E.g. if a NIC has access to guest private data then there should never be a need to convert/bounce the page.
