On 5/5/23 12:31, Sean Christopherson wrote: > On Fri, May 05, 2023, Micka�l Sala�n wrote: >> >> On 05/05/2023 18:28, Sean Christopherson wrote: >>> I have no doubt that we'll need to solve performance and scaling issues with the >>> memory attributes implementation, e.g. to utilize xarray multi-range support >>> instead of storing information on a per-4KiB-page basis, but AFAICT, the core >>> idea is sound. And a very big positive from a maintenance perspective is that >>> any optimizations, fixes, etc. for one use case (CoCo vs. hardening) should also >>> benefit the other use case. >>> >>> [1] https://lore.kernel.org/all/20230311002258.852397-22-seanjc@xxxxxxxxxx >>> [2] https://lore.kernel.org/all/Y2WB48kD0J4VGynX@xxxxxxxxxx >>> [3] https://lore.kernel.org/all/Y1a1i9vbJ%2FpVmV9r@xxxxxxxxxx >> >> I agree, I used this mechanism because it was easier at first to rely on a >> previous work, but while I was working on the MBEC support, I realized that >> it's not the optimal way to do it. >> >> I was thinking about using a new special EPT bit similar to >> EPT_SPTE_HOST_WRITABLE, but it may not be portable though. What do you >> think? > > On x86, SPTEs are even more ephemeral than memslots. E.g. for historical reasons, > KVM zaps all SPTEs if _any_ memslot is deleted, which is problematic if the guest > is moving around BARs, using option ROMs, etc. > > ARM's pKVM tracks metadata in its stage-2 PTEs, i.e. doesn't need an xarray to > otrack attributes, but that works only because pKVM is more privileged than the > host kernel, and the shared vs. private memory attribute that pKVM cares about > is very, very restricted in how it can be used and changed. > > I tried shoehorning private vs. shared metadata into x86's SPTEs in the past, and > it ended up being a constant battle with the kernel, e.g. page migration, and with > KVM itself, e.g. the above memslot mess. Sorry for the delay in responding to this. I wanted to study the KVM code and fully understand your comment before responding. Yes, I quite agree with you. I will make an attempt to address this in the next version. I am working on it right now. Thanks. Madhavan
