Re: [PATCH 5/6] x86: mm: new tunable for single vs full TLB flush

[Dave Hansen <dave@sr71.net>]

On 04/24/2014 10:53 AM, Rik van Riel wrote:
>> I do agree that it's ambiguous at best.  I'll go see if anybody cares to
>> update that bit.
> 
> I suspect that IF the TLB actually uses a 2MB entry for the
> translation, a single INVLPG will work.
> 
> However, the CPU is free to cache the translations for a 2MB
> region with a bunch of 4kB entries, if it wanted to, so in
> the end we have no guarantee that an INVLPG will actually do
> the right thing...
> 
> The same is definitely true for 1GB vs 2MB entries, with
> some CPUs being capable of parsing page tables with 1GB
> entries, but having no TLB entries for 1GB translations.

I believe we _do_ have such a guarantee.  There's another bit in the SDM
that someone pointed out to me in a footnote in "4.10.4.1":

	1. If the paging structures map the linear address using a page
	larger than 4 KBytes and there are multiple TLB entries for
	that page (see Section 4.10.2.3), the instruction invalidates
	all of them.

While that's not in the easiest-to-find place in the documents, it looks
pretty clear.

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