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

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Message-ID: <0e20a86c-bc4f-2774-3cf7-4424af7becad@amd.com>
Date: Tue, 14 Jan 2025 09:23:12 -0600
From: Tom Lendacky <thomas.lendacky@....com>
To: Dave Hansen <dave.hansen@...el.com>, Rik van Riel <riel@...riel.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 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.

Thanks,
Tom

> 
> I guess the one nasty thing is that a person debugging this might not
> have a CPUID dump handy so wouldn't actually know the number of valid
> addresses that INVLPGB can take.
> 
> But otherwise, I'm not sure an explicit panic adds _much_ value here
> over an implicit one via the #GP handler. I don't know how everybody
> else feels about it, but I'm happy just depending on the #GP for now.