linux-kernel - [RFC 18/18] vfio-pci: Assume device working after liveupdate

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Date: Mon, 5 Feb 2024 12:02:03 +0000
From: James Gowans <jgowans@...zon.com>
To: <linux-kernel@...r.kernel.org>
CC: Eric Biederman <ebiederm@...ssion.com>, <kexec@...ts.infradead.org>,
	"Joerg Roedel" <joro@...tes.org>, Will Deacon <will@...nel.org>,
	<iommu@...ts.linux.dev>, Alexander Viro <viro@...iv.linux.org.uk>, "Christian
 Brauner" <brauner@...nel.org>, <linux-fsdevel@...r.kernel.org>, Paolo Bonzini
	<pbonzini@...hat.com>, Sean Christopherson <seanjc@...gle.com>,
	<kvm@...r.kernel.org>, Andrew Morton <akpm@...ux-foundation.org>,
	<linux-mm@...ck.org>, Alexander Graf <graf@...zon.com>, David Woodhouse
	<dwmw@...zon.co.uk>, "Jan H . Schoenherr" <jschoenh@...zon.de>, Usama Arif
	<usama.arif@...edance.com>, Anthony Yznaga <anthony.yznaga@...cle.com>,
	Stanislav Kinsburskii <skinsburskii@...ux.microsoft.com>,
	<madvenka@...ux.microsoft.com>, <steven.sistare@...cle.com>,
	<yuleixzhang@...cent.com>
Subject: [RFC 18/18] vfio-pci: Assume device working after liveupdate

When re-creating a VFIO device after liveupdate no desctructive actions
should be taken on it to avoid interrupting any ongoing DMA.
Specifically bus mastering should not be cleared and the device should
not be reset. Assume that reset works properly and skip over bus
mastering reset.

Ideally this would only be done for persistent devices but in this rough
RFC there currently is no mechanism at this point to easily tell if a
device is persisted or not.
---
 drivers/vfio/pci/vfio_pci_core.c | 20 +++++++++++++-------
 1 file changed, 13 insertions(+), 7 deletions(-)

diff --git a/drivers/vfio/pci/vfio_pci_core.c b/drivers/vfio/pci/vfio_pci_core.c
index 1929103ee59a..a7f56d43e0a4 100644
--- a/drivers/vfio/pci/vfio_pci_core.c
+++ b/drivers/vfio/pci/vfio_pci_core.c
@@ -480,19 +480,25 @@ int vfio_pci_core_enable(struct vfio_pci_core_device *vdev)
 			return ret;
 	}
 
-	/* Don't allow our initial saved state to include busmaster */
-	pci_clear_master(pdev);
+	if (!liveupdate) {
+		/* Don't allow our initial saved state to include busmaster */
+		pci_clear_master(pdev);
+	}
 
 	ret = pci_enable_device(pdev);
 	if (ret)
 		goto out_power;
 
-	/* If reset fails because of the device lock, fail this path entirely */
-	ret = pci_try_reset_function(pdev);
-	if (ret == -EAGAIN)
-		goto out_disable_device;
+	if (!liveupdate) {
+		/* If reset fails because of the device lock, fail this path entirely */
+		ret = pci_try_reset_function(pdev);
+		if (ret == -EAGAIN)
+			goto out_disable_device;
 
-	vdev->reset_works = !ret;
+		vdev->reset_works = !ret;
+	} else {
+		vdev->reset_works = 1;
+	}
 	pci_save_state(pdev);
 	vdev->pci_saved_state = pci_store_saved_state(pdev);
 	if (!vdev->pci_saved_state)
-- 
2.40.1