linux-kernel - Re: [PATCH 4/7] x86/xen: support nested lazy

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Message-ID: <16a63f8a-fe9f-4a65-be45-7260858734bd@arm.com>
Date: Mon, 8 Sep 2025 09:32:09 +0200
From: Kevin Brodsky <kevin.brodsky@....com>
To: Alexander Gordeev <agordeev@...ux.ibm.com>
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org,
 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>, linux-arm-kernel@...ts.infradead.org,
 linuxppc-dev@...ts.ozlabs.org, sparclinux@...r.kernel.org,
 xen-devel@...ts.xenproject.org
Subject: Re: [PATCH 4/7] x86/xen: support nested lazy_mmu sections (again)

On 05/09/2025 17:48, Alexander Gordeev wrote:
> On Thu, Sep 04, 2025 at 01:57:33PM +0100, Kevin Brodsky wrote:
> ...
>> -static void xen_enter_lazy_mmu(void)
>> +static lazy_mmu_state_t xen_enter_lazy_mmu(void)
>>  {
>> +	if (this_cpu_read(xen_lazy_mode) == XEN_LAZY_MMU)
>> +		return LAZY_MMU_NESTED;
>> +
>>  	enter_lazy(XEN_LAZY_MMU);
>> +	return LAZY_MMU_DEFAULT;
>>  }
>>  
>>  static void xen_flush_lazy_mmu(void)
>> @@ -2167,11 +2171,12 @@ static void __init xen_post_allocator_init(void)
>>  	pv_ops.mmu.write_cr3 = &xen_write_cr3;
>>  }
>>  
>> -static void xen_leave_lazy_mmu(void)
>> +static void xen_leave_lazy_mmu(lazy_mmu_state_t state)
>>  {
>>  	preempt_disable();
>>  	xen_mc_flush();
>> -	leave_lazy(XEN_LAZY_MMU);
>> +	if (state != LAZY_MMU_NESTED)
>> +		leave_lazy(XEN_LAZY_MMU);
> Based on xen_enter_lazy_mmu(), whether this condition needs to be
> executed with the preemption disabled?

AFAIU xen_mc_flush() needs preemption to be disabled. I don't think
{enter,leave}_lazy() do, but this patch doesn't introduce any change
from that perspective. I suppose it doesn't hurt that
xen_leave_lazy_mmu() calls leave_lazy() with preemption disabled.

> Or may be this_cpu_read(xen_lazy_mode) + enter_lazy(XEN_LAZY_MMU)
> should be executed with the preemption disabled?

Adding another this_cpu_read(xen_lazy_mode) in xen_enter_lazy_mmu()
shouldn't change the situation, i.e. preemption should still be safe. If
preemption occurs in the middle of that function,
xen_{start,end}_context_switch() will do the right thing to save/restore
xen_lazy_mode.

- Kevin