Re: [PATCH][kvmtool] virtio/pci: Correctly handle MSI-X masking while MSI-X is disabled

[Marc Zyngier <maz@kernel.org>]

On Wed, 25 Aug 2021 16:33:21 +0100,
Marc Zyngier <maz@kernel.org> wrote:
> 
> On Tue, 24 Aug 2021 15:32:53 +0100,
> Marc Zyngier <maz@kernel.org> wrote:
> > 
> > Hi Andre,
> > 
> > On Mon, 23 Aug 2021 17:48:33 +0100,
> > Andre Przywara <andre.przywara@arm.com> wrote:
> > > 
> > > On Sat, 21 Aug 2021 13:07:42 +0100
> > > Marc Zyngier <maz@kernel.org> wrote:
> > > 
> > > Hi Marc,
> > > 
> > > > Since Linux commit 7d5ec3d36123 ("PCI/MSI: Mask all unused MSI-X
> > > > entries"), kvmtool segfaults when the guest boots and tries to
> > > > disable all the MSI-X entries of a virtio device while MSI-X itself
> > > > is disabled.
> > > > 
> > > > What Linux does is seems perfectly correct. However, kvmtool uses
> > > > a different decoding depending on whether MSI-X is enabled for
> > > > this device or not. Which seems pretty wrong.
> > > 
> > > While I really wish this would be wrong, I think this is
> > > indeed how this is supposed to work: The Virtio legacy spec makes the
> > > existence of those two virtio config fields dependent on the
> > > (dynamic!) enablement status of MSI-X. This is reflected in:
> > > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/include/uapi/linux/virtio_pci.h#n72
> > > and explicitly mentioned as a footnote in the virtio 0.9.5 spec[1]:
> > > "3) ie. once you enable MSI-X on the device, the other fields move. If
> > > you turn it off again, they move back!"
> > 
> > Madness! What was Rusty on at the time? I really hope the bitcoin
> > thing is buying him better stuff...
> > 
> > > I agree that this looks like a bad idea, but I am afraid we are stuck
> > > with this. It looks like the Linux driver is at fault here, it should
> > > not issue the config access when MSIs are disabled. Something like this
> > > (untested):
> > > 
> > > --- a/drivers/virtio/virtio_pci_legacy.c
> > > +++ b/drivers/virtio/virtio_pci_legacy.c
> > > @@ -103,6 +103,9 @@ static void vp_reset(struct virtio_device *vdev)
> > >  
> > >  static u16 vp_config_vector(struct virtio_pci_device *vp_dev, u16 vector)
> > >  {
> > > +       if (!vp_dev->msix_enabled)
> > > +               return VIRTIO_MSI_NO_VECTOR;
> > > +
> > >         /* Setup the vector used for configuration events */
> > >         iowrite16(vector, vp_dev->ioaddr + VIRTIO_MSI_CONFIG_VECTOR);
> > >         /* Verify we had enough resources to assign the vector */
> > > 
> > > This is just my first idea after looking at this, happy to stand
> > > corrected or hear about a better solution.
> > 
> > I don't think this works. It instead completely disables MSI-X, which
> > is a total bore. I think the only way to deal with it is to quirk it
> > to prevent the bulk masking to take effect before MSI-X is enabled.
> 
> Actually, let me correct myself. I tested the wrong configuration (why
> isn't --force-pci the bloody default in kvmtool?). This patch doesn't
> fix anything at all, and kvmtool just explodes.
> 
> Having dug further, it isn't the config space that causes problems,
> but the programming of the MSI-X vectors. I'm starting to suspect the
> layout of the MSI-X bar in kvmtool.

OK, this is hilarious. Sort of. The MSI-X bar sizing is bonkers: you
can't fit 33 MSIs there (33 being the number of MSI-X that kvmtool
advertises), and you will have notionally overwritten the PBA as
well. Amusingly, the last write ends up being misdecoded as a config
space access...

"works for me".

	M.

From a2b3a338aab535a1683cc5b424455ed7fd3a500a Mon Sep 17 00:00:00 2001
From: Marc Zyngier <maz@kernel.org>
Date: Wed, 25 Aug 2021 18:19:27 +0100
Subject: [PATCH] virtio/pci: Size the MSI-X bar according to the number of
 MSI-X

Since 45d3b59e8c45 ("kvm tools: Increase amount of possible interrupts
per PCI device"), the number of MSI-S has gone from 4 to 33.

However, the corresponding storage hasn't been upgraded, and writing
to the MSI-X table is a pretty risky business. Now that the Linux
kernel writes to *all* MSI-X entries before doing anything else
with the device, kvmtool dies a horrible death.

Fix it by properly defining the size of the MSI-X bar, and make
Linux great again.

Signed-off-by: Marc Zyngier <maz@kernel.org>
---
 virtio/pci.c | 29 +++++++++++++++++++++--------
 1 file changed, 21 insertions(+), 8 deletions(-)

diff --git a/virtio/pci.c b/virtio/pci.c
index eb91f512..726146fc 100644
--- a/virtio/pci.c
+++ b/virtio/pci.c
@@ -7,6 +7,7 @@
 #include "kvm/irq.h"
 #include "kvm/virtio.h"
 #include "kvm/ioeventfd.h"
+#include "kvm/util.h"
 
 #include <sys/ioctl.h>
 #include <linux/virtio_pci.h>
@@ -14,6 +15,13 @@
 #include <assert.h>
 #include <string.h>
 
+#define ALIGN_UP(x, s)		ALIGN((x) + (s) - 1, (s))
+#define VIRTIO_NR_MSIX		(VIRTIO_PCI_MAX_VQ + VIRTIO_PCI_MAX_CONFIG)
+#define VIRTIO_MSIX_TABLE_SIZE	(VIRTIO_NR_MSIX * 16)
+#define VIRTIO_MSIX_PBA_SIZE	(ALIGN_UP(VIRTIO_MSIX_TABLE_SIZE, 64) / 8)
+#define VIRTIO_MSIX_BAR_SIZE	(1UL << fls_long(VIRTIO_MSIX_TABLE_SIZE + \
+						 VIRTIO_MSIX_PBA_SIZE))
+
 static u16 virtio_pci__port_addr(struct virtio_pci *vpci)
 {
 	return pci__bar_address(&vpci->pci_hdr, 0);
@@ -336,15 +344,20 @@ static void virtio_pci__msix_mmio_callback(struct kvm_cpu *vcpu,
 	int vecnum;
 	size_t offset;
 
-	if (addr > msix_io_addr + PCI_IO_SIZE) {
+	if (addr > msix_io_addr + VIRTIO_MSIX_TABLE_SIZE) {
+		/* Read access to PBA */
 		if (is_write)
 			return;
-		table  = (struct msix_table *)&vpci->msix_pba;
-		offset = addr - (msix_io_addr + PCI_IO_SIZE);
-	} else {
-		table  = vpci->msix_table;
-		offset = addr - msix_io_addr;
+		offset = addr - (msix_io_addr + VIRTIO_MSIX_TABLE_SIZE);
+		if ((offset + len) > sizeof (vpci->msix_pba))
+			return;
+		memcpy(data, (void *)&vpci->msix_pba + offset, len);
+		return;
 	}
+
+	table  = vpci->msix_table;
+	offset = addr - msix_io_addr;
+
 	vecnum = offset / sizeof(struct msix_table);
 	offset = offset % sizeof(struct msix_table);
 
@@ -520,7 +533,7 @@ int virtio_pci__init(struct kvm *kvm, void *dev, struct virtio_device *vdev,
 
 	port_addr = pci_get_io_port_block(PCI_IO_SIZE);
 	mmio_addr = pci_get_mmio_block(PCI_IO_SIZE);
-	msix_io_block = pci_get_mmio_block(PCI_IO_SIZE * 2);
+	msix_io_block = pci_get_mmio_block(VIRTIO_MSIX_BAR_SIZE);
 
 	vpci->pci_hdr = (struct pci_device_header) {
 		.vendor_id		= cpu_to_le16(PCI_VENDOR_ID_REDHAT_QUMRANET),
@@ -543,7 +556,7 @@ int virtio_pci__init(struct kvm *kvm, void *dev, struct virtio_device *vdev,
 		.capabilities		= (void *)&vpci->pci_hdr.msix - (void *)&vpci->pci_hdr,
 		.bar_size[0]		= cpu_to_le32(PCI_IO_SIZE),
 		.bar_size[1]		= cpu_to_le32(PCI_IO_SIZE),
-		.bar_size[2]		= cpu_to_le32(PCI_IO_SIZE*2),
+		.bar_size[2]		= cpu_to_le32(VIRTIO_MSIX_BAR_SIZE),
 	};
 
 	r = pci__register_bar_regions(kvm, &vpci->pci_hdr,
-- 
2.30.2


-- 
Without deviation from the norm, progress is not possible.