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[PATCH v2 04/24] hw/xen: don't clear map_track[] in xen_gnttab_reset()

David Woodhouse posted 24 patches 1 year, 1 month ago

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Maintainers: Kevin Wolf <kwolf@redhat.com>, Hanna Reitz <hreitz@redhat.com>, Stefano Stabellini <sstabellini@kernel.org>, Anthony Perard <anthony.perard@citrix.com>, Paul Durrant <paul@xen.org>, "Marc-André Lureau" <marcandre.lureau@redhat.com>, Paolo Bonzini <pbonzini@redhat.com>, Richard Henderson <richard.henderson@linaro.org>, Eduardo Habkost <eduardo@habkost.net>, "Michael S. Tsirkin" <mst@redhat.com>, Marcel Apfelbaum <marcel.apfelbaum@gmail.com>, David Woodhouse <dwmw2@infradead.org>, Jason Wang <jasowang@redhat.com>, Marcelo Tosatti <mtosatti@redhat.com>, Cleber Rosa <crosa@redhat.com>, "Philippe Mathieu-Daudé" <philmd@linaro.org>, Wainer dos Santos Moschetta <wainersm@redhat.com>, Beraldo Leal <bleal@redhat.com>

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[PATCH v2 04/24] hw/xen: don't clear map_track[] in xen_gnttab_reset()

Posted by David Woodhouse 1 year, 1 month ago

From: David Woodhouse <dwmw@amazon.co.uk>

The refcounts actually correspond to 'active_ref' structures stored in a
GHashTable per "user" on the backend side (mostly, per XenDevice).

If we zero map_track[] on reset, then when the backend drivers get torn
down and release their mapping we hit the assert(s->map_track[ref] != 0)
in gnt_unref().

So leave them in place. Each backend driver will disconnect and reconnect
as the guest comes back up again and reconnects, and it all works out OK
in the end as the old refs get dropped.

Fixes: de26b2619789 ("hw/xen: Implement soft reset for emulated gnttab")
Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
---
 hw/i386/kvm/xen_gnttab.c | 2 --
 1 file changed, 2 deletions(-)

diff --git a/hw/i386/kvm/xen_gnttab.c b/hw/i386/kvm/xen_gnttab.c
index 21c30e3659..839ec920a1 100644
--- a/hw/i386/kvm/xen_gnttab.c
+++ b/hw/i386/kvm/xen_gnttab.c
@@ -541,7 +541,5 @@ int xen_gnttab_reset(void)
     s->entries.v1[GNTTAB_RESERVED_XENSTORE].flags = GTF_permit_access;
     s->entries.v1[GNTTAB_RESERVED_XENSTORE].frame = XEN_SPECIAL_PFN(XENSTORE);
 
-    memset(s->map_track, 0, s->max_frames * ENTRIES_PER_FRAME_V1);
-
     return 0;
 }
-- 
2.40.1

Re: [PATCH v2 04/24] hw/xen: don't clear map_track[] in xen_gnttab_reset()

Posted by Paul Durrant 1 year, 1 month ago

On 19/10/2023 16:40, David Woodhouse wrote:
> From: David Woodhouse <dwmw@amazon.co.uk>
> 
> The refcounts actually correspond to 'active_ref' structures stored in a
> GHashTable per "user" on the backend side (mostly, per XenDevice).
> 
> If we zero map_track[] on reset, then when the backend drivers get torn
> down and release their mapping we hit the assert(s->map_track[ref] != 0)
> in gnt_unref().
> 
> So leave them in place. Each backend driver will disconnect and reconnect
> as the guest comes back up again and reconnects, and it all works out OK
> in the end as the old refs get dropped.
> 
> Fixes: de26b2619789 ("hw/xen: Implement soft reset for emulated gnttab")
> Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
> ---
>   hw/i386/kvm/xen_gnttab.c | 2 --
>   1 file changed, 2 deletions(-)
> 

Reviewed-by: Paul Durrant <paul@xen.org>