Re: [Xen-devel] [PATCH v2 1/7] x86: slightly reduce Meltdown band-aid overhead

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 07/02/18 16:12, Jan Beulich wrote:
> I'm not sure why I didn't do this right away: By avoiding the use of
> global PTEs in the cloned directmap, there's no need to fiddle with
> CR4.PGE on any of the entry paths. Only the exit paths need to flush
> global mappings.
>
> The reduced flushing, however, implies that we now need to have
> interrupts off on all entry paths until after the page table switch, so
> that flush IPIs can't arrive with the restricted page tables still
> active, but only a non-global flush happening with the CR3 loads. Along
> those lines the "sync" IPI after L4 entry updates now needs to become a
> real (and global) flush IPI, so that inside Xen we'll also pick up such
> changes.

Actually, on second consideration, why does reenabling interrupts need
to be deferred?

The safety of the sync_guest path (which previously entered Xen, did
nothing, and exited again) relied on the entry part flushing global
mappings for safety, as the return-to-xen path doesn't necessarily
switch mappings.

However, the first hunk upgrading the "do nothing" to a proper global
flush, covers that case.

I don't see anything else which affects the safety of taking TLB flush
IPIs early in the entry-from-guest path.  What am I missing?

~Andrew

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