linux-kernel - Re: [PATCH 04/21] x86/mm/asi: set up asi_nonsensitive

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Message-ID: <0dbdd1ff-bf23-4306-bace-97f745800c13@intel.com>
Date: Wed, 12 Nov 2025 11:39:40 -0800
From: Dave Hansen <dave.hansen@...el.com>
To: Brendan Jackman <jackmanb@...gle.com>, Andy Lutomirski <luto@...nel.org>,
 Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 "Liam R. Howlett" <Liam.Howlett@...cle.com>,
 Suren Baghdasaryan <surenb@...gle.com>, Michal Hocko <mhocko@...e.com>,
 Johannes Weiner <hannes@...xchg.org>, Zi Yan <ziy@...dia.com>,
 Axel Rasmussen <axelrasmussen@...gle.com>, Yuanchu Xie <yuanchu@...gle.com>,
 Roman Gushchin <roman.gushchin@...ux.dev>
Cc: peterz@...radead.org, bp@...en8.de, dave.hansen@...ux.intel.com,
 mingo@...hat.com, tglx@...utronix.de, akpm@...ux-foundation.org,
 david@...hat.com, derkling@...gle.com, junaids@...gle.com,
 linux-kernel@...r.kernel.org, linux-mm@...ck.org, reijiw@...gle.com,
 rientjes@...gle.com, rppt@...nel.org, vbabka@...e.cz, x86@...nel.org,
 yosry.ahmed@...ux.dev
Subject: Re: [PATCH 04/21] x86/mm/asi: set up asi_nonsensitive_pgd

On 10/2/25 10:19, Brendan Jackman wrote:
> On Thu Oct 2, 2025 at 4:14 PM UTC, Dave Hansen wrote:
...>> What is the point of having a pgd if you can't put it in CR3? If you:
>>
>> 	write_cr3(asi_nonsensitive_pgd);
>>
>> you'll just triple fault because all kernel text is !_PAGE_PRESENT.
>>
>> The critical point is when 'asi_nonsensitive_pgd' is functional enough
>> that it can be loaded into CR3 and handle a switch to the normal
>> init_mm->pgd.
> 
> Hm, are you saying that I should expand the scope of the patchset from
> "set up the direct map" to "set up an ASI address space"? If so, yeah I
> can do that, I don't think the patchset would get that much bigger. I
> only left the other bits out because it feels weird to set up a whole
> address space but never actually switch into it. Setting up the logic to
> switch into it would make the patchset really big though.

The patch set has to _do_ something, though. It's fine for a patch
series to add code that then gets turned on at the end of the series.
But, at the end of the series, it has to have something to show for it.

If the series is small *and* useful, all the better. But, if I have to
choose between small or useful, it's always going to be useful.

> Like I said in the cover letter, I could also always change tack:
> we could instead start with all the address-space switching logic, but
> just have the two address spaces be clones of each other. Then we could
> come back and start poking holes in the ASI one for the second series. I
> don't have a really strong opinion about the best place to start, but
> I'll stick to my current course unless someone else does have a strong
> opinion.

Yeah, but the end of the series has to have holes poked that are
marginally useful for *SOMETHING*, at least if anyone wants it applied.