linux-kernel - Re: [PATCH v4 02/12] x86/xen: simplify flush_lazy

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Message-ID: <b21fb5b9-2e7f-4cbc-ae62-015b1317a9bd@arm.com>
Date: Mon, 10 Nov 2025 11:36:52 +0100
From: Kevin Brodsky <kevin.brodsky@....com>
To: Ryan Roberts <ryan.roberts@....com>, linux-mm@...ck.org
Cc: linux-kernel@...r.kernel.org, Alexander Gordeev <agordeev@...ux.ibm.com>,
 Andreas Larsson <andreas@...sler.com>,
 Andrew Morton <akpm@...ux-foundation.org>,
 Boris Ostrovsky <boris.ostrovsky@...cle.com>, Borislav Petkov
 <bp@...en8.de>, Catalin Marinas <catalin.marinas@....com>,
 Christophe Leroy <christophe.leroy@...roup.eu>,
 Dave Hansen <dave.hansen@...ux.intel.com>,
 David Hildenbrand <david@...hat.com>, "David S. Miller"
 <davem@...emloft.net>, David Woodhouse <dwmw2@...radead.org>,
 "H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
 Jann Horn <jannh@...gle.com>, Juergen Gross <jgross@...e.com>,
 "Liam R. Howlett" <Liam.Howlett@...cle.com>,
 Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 Madhavan Srinivasan <maddy@...ux.ibm.com>,
 Michael Ellerman <mpe@...erman.id.au>, Michal Hocko <mhocko@...e.com>,
 Mike Rapoport <rppt@...nel.org>, Nicholas Piggin <npiggin@...il.com>,
 Peter Zijlstra <peterz@...radead.org>, Suren Baghdasaryan
 <surenb@...gle.com>, Thomas Gleixner <tglx@...utronix.de>,
 Vlastimil Babka <vbabka@...e.cz>, Will Deacon <will@...nel.org>,
 Yeoreum Yun <yeoreum.yun@....com>, linux-arm-kernel@...ts.infradead.org,
 linuxppc-dev@...ts.ozlabs.org, sparclinux@...r.kernel.org,
 xen-devel@...ts.xenproject.org, x86@...nel.org
Subject: Re: [PATCH v4 02/12] x86/xen: simplify flush_lazy_mmu()

On 07/11/2025 12:31, Ryan Roberts wrote:
> On 29/10/2025 10:08, Kevin Brodsky wrote:
>> arch_flush_lazy_mmu_mode() is called when outstanding batched
>> pgtable operations must be completed immediately. There should
>> however be no need to leave and re-enter lazy MMU completely. The
>> only part of that sequence that we really need is xen_mc_flush();
>> call it directly.
>>
>> Signed-off-by: Kevin Brodsky <kevin.brodsky@....com>
> This looks functionally equivalent to me, so:
>
> Reviewed-by: Ryan Roberts <ryan.roberts@....com>
>
> But I don't think this tidy up is strictly necessary for your series to work?
> (perhaps I'll change my mind on that as I go through it).

I initially thought it might be, but in the end I think you're right -
it should still work fine without this patch.

Still, I'd rather avoid unnecessary calls to arch_enter() and
arch_leave() as it makes it harder to reason about what is called where.
Namely, keeping them here means that a nested call to
lazy_mmu_mode_disable() would cause arch_leave() then arch_enter() to be
called - rather unexpected.

The only calls to arch_enter() and arch_leave() that are left after this
series are the ones in <linux/pgtable.h> and the Xen context-switching
logic (the one case where calling arch hooks directly is justified, see
discussion on v3 [1]).

- Kevin

[1]
https://lore.kernel.org/all/390e41ae-4b66-40c1-935f-7a1794ba0b71@arm.com/