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Sampat wrote: >> >> >> On 1/13/26 11:53 AM, Kiryl Shutsemau wrote: >>> On Tue, Jan 13, 2026 at 11:10:21AM -0600, Pratik R. Sampat wrote: >>>> >>>> >>>> On 1/13/2026 4:28 AM, Kiryl Shutsemau wrote: >>>>> On Mon, Jan 12, 2026 at 02:23:00PM -0600, Pratik R. Sampat wrote: >>>>>> Transition memory to the shared state during a hot-remove operation so >>>>>> that it can be re-used by the hypervisor. This also applies when memory >>>>>> is intended to be hotplugged back in later, as those pages will need to >>>>>> be re-accepted after crossing the trust boundary. >>>>> >>>>> Hm. What happens when we hot-remove memory that was there at the boot >>>>> and there's bitmap space for it? >>>>> >>>> >>>> While hotplug ranges gotten from SRAT don't seem to overlap with the >>>> conventional ranges in the unaccepted table, EFI_MEMORY_HOT_PLUGGABLE >>>> attribute could indicate boot time memory that could be hot-removed. I >>>> could potentially unset the bitmap first, if the bit exists and then >>>> unaccept. >>>> >>>> Similarly, I could also check if the bitmap is large enough to set the >>>> bit before I call arch_accept_memory() (This may not really be needed >>>> though). >>>> >>>>> Also, I'm not sure why it is needed. At least in TDX case, VMM can pull >>>>> the memory from under guest at any time without a warning. Coverting >>>>> memory to shared shouldn't make a difference as along as re-adding the >>>>> same GPA range triggers accept. >>>>> >>>> >>>> That makes sense. The only scenario where we could run into trouble on >>>> SNP platforms is when we redo a qemu device_add after a device_del >>>> without first removing the memory object entirely since same-state >>>> transitions result in guest termination. >>>> >>>> This means we must always follow a device_del with an object_del on >>>> removal. Otherwise, the onus would then be on the VMM to transition >>>> the memory back to shared before re-adding it to the guest. >>> >>> This seems to be one-of-many possible ways of VMM to get guest terminated. >>> DoS is not in something confidential computing aims to prevent. >>> >>>> However, if this flow is not a concern to begin with then I could >>>> probably just drop this patch? >>> >>> Yes, please. >> >> Putting more thought into it, memory unacceptance on remove may be required >> after all at least for SNP platforms. >> >> Consider a scenario: >> * Guest accepts a GPA say G1, mapped to a host physical address H1. >> * We attempt to hot-remove the memory. If the guest does not unaccept the memory >> now then G1 to H1 mapping within the RMP will still exist. >> * Then if the hypervisor later hot-adds the memory to G1, it will be now mapped >> to H3 and this new mapping will be accepted. >> >> This will essentially mean that we have 2 RMP entries: One for H1 and another >> for H3 mapped for G1 which are both validated / accepted which can then be >> swapped at will and compromise integrity. > > I don't know much about SEV, but I assume RMP is similar to PAMT in TDX > where TDX module maintains metadata for host physical memory. > > What side problems do you for guest here? > > I probably miss something, but it seems to be VMM problem, no? I mean if > VMM doesn't update RMP on replacing one HPA to another for the GPA, it > is bug in VMM housekeeping. Guest is not responsible for this. > Right, the problem is that we do not inherently trust the host to make change. That is why in my understanding, the guest is responsible for validating and rescinding those pages. --Pratik