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Re: [PATCH 1/3] x86/shadow: unlock P2M slightly earlier in shadow_track_dirty_vram()
- To: Jan Beulich <jbeulich@xxxxxxxx>
- From: Roger Pau Monné <roger.pau@xxxxxxxxxx>
- Date: Mon, 27 Apr 2026 10:47:23 +0200
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- Cc: "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>, Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- Delivery-date: Mon, 27 Apr 2026 08:47:35 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On Tue, Feb 03, 2026 at 05:49:35PM +0100, Jan Beulich wrote:
> There's no need to call vfree() with the lock still held.
>
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
Acked-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
> ---
> In fact for the purposes of the function the P2M lock could really be
> obtained merely in read mode, and it could be dropped immediately in both
> the main "if()" and its "else if()". If only there wasn't the error
> handling after copy_to_guest(): Dropping the paging lock ahead of that
> call, we rely solely on the P2M lock to also guard the changing of
> d->arch.hvm.dirty_vram.sh and what it points to. Question is why dropping
> the paging lock (but continuing to hold the P2M lock) is necessary there
> in the first place.
I wouldn't spend a lot of time trying to optimize this, we already
know HVM shadow is ATM not very optimized, and generally recommend
HAP.
If we could turn the p2m lock into read-mode, maybe at the expense of
expanding the paging locked region that would likely be slightly better?
Thanks, Roger.
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