linux-kernel - Re: [PATCH v3 02/13] x86/xen: simplify flush_lazy

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Message-ID: <54f183bb-33ce-4b9c-91a9-827a6ed198d6@arm.com>
Date: Thu, 16 Oct 2025 09:32:13 +0200
From: Kevin Brodsky <kevin.brodsky@....com>
To: Dave Hansen <dave.hansen@...el.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>, "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>, Ryan Roberts <ryan.roberts@....com>,
 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 v3 02/13] x86/xen: simplify flush_lazy_mmu()

On 15/10/2025 18:52, Dave Hansen wrote:
> On 10/15/25 01:27, Kevin Brodsky wrote:
>> While at it, we can also avoid preempt_disable() if we are not
>> in lazy MMU mode - xen_get_lazy_mode() should tolerate preemption.
> ...
>>  static void xen_flush_lazy_mmu(void)
>>  {
>> -	preempt_disable();
>> -
>>  	if (xen_get_lazy_mode() == XEN_LAZY_MMU) {
>> -		arch_leave_lazy_mmu_mode();
>> -		arch_enter_lazy_mmu_mode();
>> +		preempt_disable();
>> +		xen_mc_flush();
>> +		preempt_enable();
>>  	}
> But xen_get_lazy_mode() does:
>
> 	this_cpu_read(xen_lazy_mode);
>
> Couldn't preemption end up doing the 'xen_lazy_mode' read and the
> xen_mc_flush() on different CPUs?
>
> That seems like a problem. Is there a reason it's safe?

You're right, I was thinking in terms of task, but xen_mc_flush() does
operate on the current CPU (and it's called when context-switching).
Will restore the original order in v4.

- Kevin