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    SubjectRe: [RFC][PATCH 08/24] x86/entry: Add C version of SWAPGS and SWAPGS_UNSAFE_STACK
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    [Copying the reply to Andy in the thread with the right email addresses]

    On 11/9/20 6:38 PM, Andy Lutomirski wrote:
    > On Mon, Nov 9, 2020 at 3:22 AM Alexandre Chartre
    > <alexandre.chartre@oracle.com> wrote:
    >>
    >> SWAPGS and SWAPGS_UNSAFE_STACK are assembly macros. Add C versions
    >> of these macros (swapgs() and swapgs_unsafe_stack()).
    >
    > This needs a very good justification. It also needs some kind of
    > static verification that these helpers are only used by noinstr code,
    > and they need to be __always_inline. And I cannot fathom how C code
    > could possibly use SWAPGS_UNSAFE_STACK in a meaningful way.
    >

    You're right, I probably need to revisit the usage of SWAPGS_UNSAFE_STACK
    in C code, that doesn't make sense. Looks like only SWAPGS is then needed.

    Or maybe we can just use native_swapgs() instead?

    I have added a C version of SWAPGS for moving paranoid_entry() to C because,
    in this function, we need to switch CR3 before doing the updating GS. But I
    really wonder if we need a paravirt swapgs here, and we can probably just use
    native_swapgs().

    Also, if we map the per cpu offsets (__per_cpu_offset) in the user page-table
    then we will be able to update GS before switching CR3. That way we can keep the
    GS update in assembly code, and just do the CR3 switch in C code. This would also
    avoid having to disable stack-protector (patch 21).

    alex.

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