linux-kernel - Re: [PATCH v4 05/12] x86/mm: add INVLPGB support code

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Message-ID: <fa9b69251760c832d86c3d3fc7a3e349e2f8b733.camel@surriel.com>
Date: Tue, 14 Jan 2025 10:47:22 -0500
From: Rik van Riel <riel@...riel.com>
To: Tom Lendacky <thomas.lendacky@....com>, Dave Hansen
 <dave.hansen@...el.com>, 	x86@...nel.org
Cc: linux-kernel@...r.kernel.org, bp@...en8.de, peterz@...radead.org, 
	dave.hansen@...ux.intel.com, zhengqi.arch@...edance.com,
 nadav.amit@...il.com, 	kernel-team@...a.com, linux-mm@...ck.org,
 akpm@...ux-foundation.org, 	jannh@...gle.com
Subject: Re: [PATCH v4 05/12] x86/mm: add INVLPGB support code

On Tue, 2025-01-14 at 09:23 -0600, Tom Lendacky wrote:
> On 1/14/25 09:05, Dave Hansen wrote:
> > On 1/14/25 06:29, Tom Lendacky wrote:
> > > > Given the choice between "a bug in the calling code
> > > > crashes the kernel" and "a bug in the calling code
> > > > results in a missed TLB flush", I'm guessing the
> > > > crash is probably better.
> > > So instead of the negative number protection, shouldn't this just
> > > use an
> > > unsigned int for extra_count and panic() if the value is greater
> > > than
> > > invlpgb_count_max? The caller has some sort of logic problem and
> > > it
> > > could possibly result in missed TLB flushes. Or if a panic() is
> > > out of
> > > the question, maybe a WARN() and a full TLB flush to be safe?
> > 
> > The current implementation will panic in the #GP handler though. It
> > should be pretty easy to figure out that INVLPGB is involved with
> > RIP or
> > the Code: snippet. From there, you'd need to figure out what caused
> > the #GP.
> 
> Hmmm, maybe I'm missing something. IIUC, when a negative number is
> supplied, the extra_count field will be set to 0 (via the max()
> function) and allow the INVLPGB to continue. 0 is valid in ECX[15:0]
> and
> so the instruction won't #GP.

I added that at the request of somebody else :)

Let me remove it again, now that we seem to have a
consensus that a panic is preferable to a wrong
TLB flush.

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