Messages in this thread | | | Date | Thu, 2 Nov 2023 16:48:41 +0100 | Subject | Re: [PATCH v13 16/35] KVM: Add KVM_CREATE_GUEST_MEMFD ioctl() for guest-specific backing memory | From | Paolo Bonzini <> |
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On 10/31/23 23:39, David Matlack wrote: >>> Maybe can you sketch out how you see this proposal being extensible to >>> using guest_memfd for shared mappings? >> For in-place conversions, e.g. pKVM, no additional guest_memfd is needed. What's >> missing there is the ability to (safely) mmap() guest_memfd, e.g. KVM needs to >> ensure there are no outstanding references when converting back to private. >> >> For TDX/SNP, assuming we don't find a performant and robust way to do in-place >> conversions, a second fd+offset pair would be needed. > Is there a way to support non-in-place conversions within a single guest_memfd?
For TDX/SNP, you could have a hook from KVM_SET_MEMORY_ATTRIBUTES to guest memory. The hook would invalidate now-private parts if they have a VMA, causing a SIGSEGV/EFAULT if the host touches them.
It would forbid mappings from multiple gfns to a single offset of the guest_memfd, because then the shared vs. private attribute would be tied to the offset. This should not be a problem; for example, in the case of SNP, the RMP already requires a single mapping from host physical address to guest physical address.
Paolo
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