linux-kernel - Re: [PATCH v3 11/13] x86/xen: use lazy_mmu

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Message-ID: <fcd7b731d38b256e59edd532e792a00efa4e144e.camel@physik.fu-berlin.de>
Date: Fri, 24 Oct 2025 17:38:26 +0200
From: John Paul Adrian Glaubitz <glaubitz@...sik.fu-berlin.de>
To: David Woodhouse <dwmw2@...radead.org>, David Hildenbrand
 <david@...hat.com>,  Kevin Brodsky <kevin.brodsky@....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 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 11/13] x86/xen: use lazy_mmu_state when
 context-switching

On Fri, 2025-10-24 at 16:13 +0100, David Woodhouse wrote:
> On Fri, 2025-10-24 at 16:51 +0200, David Hildenbrand wrote:
> > On 24.10.25 16:47, David Woodhouse wrote:
> > > On Thu, 2025-10-23 at 22:06 +0200, David Hildenbrand wrote:
> > > > On 15.10.25 10:27, Kevin Brodsky wrote:
> > > > > We currently set a TIF flag when scheduling out a task that is in
> > > > > lazy MMU mode, in order to restore it when the task is scheduled
> > > > > again.
> > > > > 
> > > > > The generic lazy_mmu layer now tracks whether a task is in lazy MMU
> > > > > mode in task_struct::lazy_mmu_state. We can therefore check that
> > > > > state when switching to the new task, instead of using a separate
> > > > > TIF flag.
> > > > > 
> > > > > Signed-off-by: Kevin Brodsky <kevin.brodsky@....com>
> > > > > ---
> > > > 
> > > > 
> > > > Looks ok to me, but I hope we get some confirmation from x86 / xen
> > > > folks.
> > > 
> > > 
> > > I know tglx has shouted at me in the past for precisely this reminder,
> > > but you know you can test Xen guests under QEMU/KVM now and don't need
> > > to actually run Xen? Has this been boot tested?
> > 
> > And after that, boot-testing sparc as well? :D
> 
> Also not that hard in QEMU, I believe. Although I do have some SPARC
> boxes in the shed...

Please have people test kernel changes on SPARC on real hardware. QEMU does not
emulate sun4v, for example, and therefore testing in QEMU does not cover all
of SPARC hardware.

There are plenty of people on the debian-sparc, gentoo-sparc and sparclinux
LKML mailing lists that can test kernel patches for SPARC. If SPARC-relevant
changes need to be tested, please ask there and don't bury such things in a
deeply nested thread in a discussion which doesn't even have SPARC in the
mail subject.

Adrian

-- 
 .''`.  John Paul Adrian Glaubitz
: :' :  Debian Developer
`. `'   Physicist
  `-    GPG: 62FF 8A75 84E0 2956 9546  0006 7426 3B37 F5B5 F913