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Re: [PATCH] x86/xen: Tolerate nested XEN_LAZY_MMU entering/leaving
- To: Marek Marczykowski-Górecki <marmarek@xxxxxxxxxxxxxxxxxxxxxx>
- From: Jürgen Groß <jgross@xxxxxxxx>
- Date: Tue, 12 May 2026 18:10:10 +0200
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- Cc: linux-kernel@xxxxxxxxxxxxxxx, x86@xxxxxxxxxx, kevin.brodsky@xxxxxxx, Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>, Thomas Gleixner <tglx@xxxxxxxxxx>, Ingo Molnar <mingo@xxxxxxxxxx>, Borislav Petkov <bp@xxxxxxxxx>, Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>, "H. Peter Anvin" <hpa@xxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxxx
- Delivery-date: Tue, 12 May 2026 16:10:16 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 12.05.26 18:05, Marek Marczykowski-Górecki wrote:
On Fri, May 08, 2026 at 04:39:33PM +0200, Juergen Gross wrote:
With the support of nested lazy mmu sections it can happen that
arch_enter_lazy_mmu_mode() is being called twice without a call of
arch_leave_lazy_mmu_mode() in between, as the lazy_mmu_*() helpers
are not disabling preemption when checking for nested lazy mmu
sections.
This is a problem when running as a Xen PV guest, as
xen_enter_lazy_mmu() and xen_leave_lazy_mmu() don't tolerate this
case.
Fix that in xen_enter_lazy_mmu() and xen_leave_lazy_mmu() in order
not to hurt all other lazy mmu mode users.
Fixes: 291b3abed657 ("x86/xen: use lazy_mmu_state when context-switching")
Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
I have ran several test iterations with this patch (on top of 7.0.4) and
it seems to fix the issue. So,
Tested-by: Marek Marczykowski-Górecki <marmarek@xxxxxxxxxxxxxxxxxxxxxx>
Thanks for testing.
I did run some tests also with 291b3abed657 reverted (instead of this
patch), and that seems to work too, but I didn't run enough of
iterations to be 100% sure. Would it be helpful to that that further
too?
I do prefer my variant, as it is on my preferred path to get rid of the
Xen-private lazy mode tracking.
So in my personal opinion you don't need to continue this test.
Juergen
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