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    SubjectRe: [PATCH, RFC 44/62] x86/mm: Set KeyIDs in encrypted VMAs for MKTME
    On Fri, Jun 14, 2019 at 11:26:10AM -0700, Dave Hansen wrote:
    > On 6/14/19 10:33 AM, Alison Schofield wrote:
    > > Preserving the data across encryption key changes has not
    > > been a requirement. I'm not clear if it was ever considered
    > > and rejected. I believe that copying in order to preserve
    > > the data was never considered.
    >
    > We could preserve the data pretty easily. It's just annoying, though.
    > Right now, our only KeyID conversions happen in the page allocator. If
    > we were to convert in-place, we'd need something along the lines of:
    >
    > 1. Allocate a scratch page
    > 2. Unmap target page, or at least make it entirely read-only
    > 3. Copy plaintext into scratch page
    > 4. Do cache KeyID conversion of page being converted:
    > Flush caches, change page_ext metadata
    > 5. Copy plaintext back into target page from scratch area
    > 6. Re-establish PTEs with new KeyID

    Seems like the 'Copy plaintext' steps might disappoint the user, as
    much as the 'we don't preserve your data' design. Would users be happy
    w the plain text steps ?
    Alison

    >
    > #2 is *really* hard. It's similar to the problems that the poor
    > filesystem guys are having with RDMA these days when RDMA is doing writes.
    >
    > What we have here (destroying existing data) is certainly the _simplest_
    > semantic. We can certainly give it a different name, or even non-PROT_*
    > semantics where it shares none of mprotect()'s functionality.
    >
    > Doesn't really matter to me at all.

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