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SubjectRe: [RFC PATCH v7 00/16] Add support for eXclusive Page Frame Ownership
On Thu, Jan 10, 2019 at 03:40:04PM -0800, Dave Hansen wrote:
> Anything else you do will have *some* reduced mitigation value, which
> isn't a deal-breaker (to me at least). Some ideas:
>
> Take a look at the SWITCH_TO_KERNEL_CR3 in head_64.S. Every time that
> gets called, we've (potentially) just done a user->kernel transition and
> might benefit from flushing the TLB. We're always doing a CR3 write (on
> Meltdown-vulnerable hardware) and it can do a full TLB flush based on if
> X86_CR3_PCID_NOFLUSH_BIT is set. So, when you need a TLB flush, you
> would set a bit that ADJUST_KERNEL_CR3 would see on the next
> user->kernel transition on *each* CPU. Potentially, multiple TLB
> flushes could be coalesced this way. The downside of this is that
> you're exposed to the old TLB entries if a flush is needed while you are
> already *in* the kernel.

I would really prefer not to depend on the PTI crud for new stuff. We
really want to get rid of that code on unaffected CPUs.

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