Message-ID: <1db69026-199c-4cee-bb3b-1217f8a3afad@arm.com>
Date: Mon, 27 Oct 2025 14:38:00 +0100
From: Kevin Brodsky <kevin.brodsky@....com>
To: David Woodhouse <dwmw2@...radead.org>,
 David Hildenbrand <david@...hat.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 24/10/2025 17:17, David Woodhouse wrote:
> On Fri, 2025-10-24 at 17:05 +0200, Kevin Brodsky wrote:
>> On 24/10/2025 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?
>> I considered boot-testing a Xen guest (considering the Xen-specific
>> changes in this series), but having no idea how to go about it I quickly
>> gave up... Happy to follow instructions :)
> https://qemu-project.gitlab.io/qemu/system/i386/xen.html covers booting
> Xen HVM guests, and near the bottom PV guests too (for which you do
> need a copy of Xen to run in QEMU with '--kernel xen', and your
> distro's build should suffice for that).
>
> Let me know if you have any trouble. Here's a sample command line which
> works here...
>
> qemu-system-x86_64 -display none --accel kvm,xen-version=0x40011,kernel-irqchip=split -drive file=/var/lib/libvirt/images/fedora28.qcow2,if=xen -kernel ~/git/linux-2.6/arch/x86/boot/bzImage -append "root=/dev/xvda1 console=ttyS0" -serial mon:stdio

Thanks this is helpful! Unfortunately lazy_mmu is only used in the PV
case, so I'd need to run a PV guest. And the distro I'm using (Arch
Linux) does not have a Xen package :/ It can be built from source from
the AUR but that looks rather involved. Are there some prebuilt binaries
I could grab and just point QEMU to?

- Kevin

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