linux-kernel - Re: [PATCH] x86/mm: resize user_pcid_flush_mask for PTI / broadcast TLB flush combination

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Message-ID: <0cd34b6c8d8009fd6f626f3287ce36e55374b140.camel@surriel.com>
Date: Sat, 17 May 2025 17:29:06 -0400
From: Rik van Riel <riel@...riel.com>
To: Ingo Molnar <mingo@...nel.org>
Cc: Dave Hansen <dave.hansen@...ux.intel.com>, Andy Lutomirski
 <luto@...nel.org>,  Peter Zijlstra <peterz@...radead.org>, Ingo Molnar
 <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, 	x86@...nel.org,
 linux-kernel@...r.kernel.org, kernel-team@...a.com
Subject: Re: [PATCH] x86/mm: resize user_pcid_flush_mask for PTI / broadcast
 TLB flush combination

On Sat, 2025-05-17 at 09:59 +0200, Ingo Molnar wrote:
> 
> CONFIG_X86_TLB_BROADCAST_TLB_FLUSH doesn't actually exist, the name
> is 
> CONFIG_BROADCAST_TLB_FLUSH.
> 
Argh, cut'n'pasted from the wrong tree :(

> 
> we could make this a more obvious:
> 

> And we can drop the ugly & fragile type cast in
> invalidate_user_asid():
> 
> -	__set_bit(kern_pcid(asid),
> -		  (unsigned long
> *)this_cpu_ptr(&cpu_tlbstate.user_pcid_flush_mask));
> 
> +	__set_bit(kern_pcid(asid),
> this_cpu_ptr(cpu_tlbstate.user_pcid_flush_mask));
> 
That is a really nice improvement, and it almost
works, too ;)

In file included from ./arch/x86/include/asm/bitops.h:430,
                 from ./include/linux/bitops.h:68:
./include/asm-generic/bitops/instrumented-non-atomic.h:26:54: note:
expected ‘volatile long unsigned int *’ but argument is of type ‘long
unsigned int (*)[32]’
   26 | ___set_bit(unsigned long nr, volatile unsigned long *addr)
      |                              ~~~~~~~~~~~~~~~~~~~~~~~~^~~~


I ended up settling for this:

      __set_bit(kern_pcid(asid),
this_cpu_ptr(&cpu_tlbstate.user_pcid_flush_mask[0]));

> 3)
> 
> If we are going to grow user_pcid_flush_mask from 2 bytes to 256
> bytes 
> then please reorder 'struct tlb_state' for cache efficiency: at
> minimum 
> the ::cr4 shadow should move to before ::user_pcid_flush_mask. But I 
> think we should probably move user_pcid_flush_mask to the end of the 
> structure, where it does the least damage to cache layout.

Done. V2 to follow in another email.

Thank you!

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