linux-kernel - Re: [PATCH 1/3] x86/mm: Correct the actual count of available global ASIDs

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Message-ID: <20250331034626.GA88569@k08j02272.eu95sqa>
Date: Mon, 31 Mar 2025 11:46:26 +0800
From: "Hou Wenlong" <houwenlong.hwl@...group.com>
To: Rik van Riel <riel@...riel.com>
Cc: linux-kernel@...r.kernel.org, Dave Hansen <dave.hansen@...ux.intel.com>,
	Andy Lutomirski <luto@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
	x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH 1/3] x86/mm: Correct the actual count of available global
 ASIDs

On Sun, Mar 30, 2025 at 01:29:25AM +0800, Rik van Riel wrote:
> On Sat, 2025-03-29 at 21:05 +0800, Hou Wenlong wrote:
> > 
> > +++ b/arch/x86/mm/tlb.c
> > @@ -279,7 +279,7 @@ static DEFINE_RAW_SPINLOCK(global_asid_lock);
> >  static u16 last_global_asid = MAX_ASID_AVAILABLE;
> >  static DECLARE_BITMAP(global_asid_used, MAX_ASID_AVAILABLE);
> >  static DECLARE_BITMAP(global_asid_freed, MAX_ASID_AVAILABLE);
> > -static int global_asid_available = MAX_ASID_AVAILABLE -
> > TLB_NR_DYN_ASIDS - 1;
> > +static int global_asid_available = MAX_ASID_AVAILABLE -
> > TLB_NR_DYN_ASIDS;
> 
> Unfortunately we are limited by the PCID space.
> 
> A process with ASID N will get PCID N+1.
> 
> The PCID space has the same size (and maximum value)
> as the ASID space.
> 
> That means we cannot use the top ASID value.
>
Thank you for your quick reply!

Since the size of the bitmap is 'MAX_ASID_AVAILABLE', the maximum ASID
value that can be allocated from the bitmap is 'MAX_ASID_AVAILABLE-1'.
This matches the comments stating that the valid ASID range for global
ASID allocation is [TLB_NR_DYN_ASIDS, MAX_ASID_AVAILABLE-1]. It is a
close interval so the size should be '(MAX_ASID_AVAILABLE-1)-TLB_NR_DYN_ASIDS+1',
regardless of the value of 'MAX_ASID_AVAILABLE'. Moreover, 'MAX_ASID_AVAILABLE'
has already taken the reserved PCID 0 into account; 'MAX_ASID_AVAILABLE-1' is a
valid ASID, and the associated PCID 'MAX_ASID_AVAILABLE' is also a valid PCID.
Did I miss something?

> Alternatively, I suppose we could have ASID and PCID
> line up, and always exclude ASID 0 from being used.
> 
> That might (maybe) be prettier code, but it isn't what
> we have today.
>
Yes, I agree. Excluding ASID 0 and using PCID as ASID makes the code
clearer in global ASID allocation.

> 
> -- 
> All Rights Reversed.