Re: [PATCH] PCI/MSI: skip masking MSI on Xen PV

[Bjorn Helgaas <helgaas@xxxxxxxxxx>]

On Thu, Aug 26, 2021 at 06:36:49PM +0200, Marek Marczykowski-Górecki wrote:
> On Thu, Aug 26, 2021 at 09:55:32AM -0500, Bjorn Helgaas wrote:
> > If/when you repost this, please run "git log --oneline
> > drivers/pci/msi.c" and follow the convention of capitalizing the
> > subject line.
> > 
> > Also, I think this patch refers specifically to MSI-X, not MSI, so
> > please update the subject line and the "masking MSI" below to reflect
> > that.
> 
> Sure, thanks for pointing this out. Is the patch otherwise ok? Should I
> post v2 with just updated commit message?

Wouldn't hurt to post a v2.  I don't have any objections to the patch,
but ultimately up to Thomas.

> > On Thu, Aug 26, 2021 at 03:43:37PM +0200, Marek Marczykowski-Górecki wrote:
> > > When running as Xen PV guest, masking MSI is a responsibility of the
> > > hypervisor. Guest has no write access to relevant BAR at all - when it
> > > tries to, it results in a crash like this:
> > > 
> > >     BUG: unable to handle page fault for address: ffffc9004069100c
> > >     #PF: supervisor write access in kernel mode
> > >     #PF: error_code(0x0003) - permissions violation
> > >     PGD 18f1c067 P4D 18f1c067 PUD 4dbd067 PMD 4fba067 PTE 80100000febd4075
> > >     Oops: 0003 [#1] SMP NOPTI
> > >     CPU: 0 PID: 234 Comm: kworker/0:2 Tainted: G        W         
> > > 5.14.0-rc7-1.fc32.qubes.x86_64 #15
> > >     Workqueue: events work_for_cpu_fn
> > >     RIP: e030:__pci_enable_msix_range.part.0+0x26b/0x5f0
> > >     Code: 2f 96 ff 48 89 44 24 28 48 89 c7 48 85 c0 0f 84 f6 01 00 00 45 
> > > 0f b7 f6 48 8d 40 0c ba 01 00 00 00 49 c1 e6 04 4a 8d 4c 37 1c <89> 10 48 
> > > 83 c0 10 48 39 c1 75 f5 41 0f b6 44 24 6a 84 c0 0f 84 48
> > >     RSP: e02b:ffffc9004018bd50 EFLAGS: 00010212
> > >     RAX: ffffc9004069100c RBX: ffff88800ed412f8 RCX: ffffc9004069105c
> > >     RDX: 0000000000000001 RSI: 00000000000febd4 RDI: ffffc90040691000
> > >     RBP: 0000000000000003 R08: 0000000000000000 R09: 00000000febd404f
> > >     R10: 0000000000007ff0 R11: ffff88800ee8ae40 R12: ffff88800ed41000
> > >     R13: 0000000000000000 R14: 0000000000000040 R15: 00000000feba0000
> > >     FS:  0000000000000000(0000) GS:ffff888018400000(0000) 
> > > knlGS:0000000000000000
> > >     CS:  e030 DS: 0000 ES: 0000 CR0: 0000000080050033
> > >     CR2: ffff8000007f5ea0 CR3: 0000000012f6a000 CR4: 0000000000000660
> > >     Call Trace:
> > >      e1000e_set_interrupt_capability+0xbf/0xd0 [e1000e]
> > >      e1000_probe+0x41f/0xdb0 [e1000e]
> > >      local_pci_probe+0x42/0x80
> > >     (...)
> > > 
> > > There is pci_msi_ignore_mask variable for bypassing MSI masking on Xen
> > > PV, but msix_mask_all() missed checking it. Add the check there too.
> > > 
> > > Fixes: 7d5ec3d36123 ("PCI/MSI: Mask all unused MSI-X entries")
> > > Cc: stable@xxxxxxxxxxxxxxx
> > 
> > 7d5ec3d36123 appeared in v5.14-rc6, so if this fix is merged before
> > v5.14, the stable tag will be unnecessary.  But we are running out of
> > time there.
> 
> 7d5ec3d36123 was already backported to stable branches (at least 5.10
> and 5.4), and in fact this is how I discovered the issue...

Oh, right, of course.  Sorry :)

> > > Signed-off-by: Marek Marczykowski-Górecki 
> > > <marmarek@xxxxxxxxxxxxxxxxxxxxxx>
> > > ---
> > > Cc: xen-devel@xxxxxxxxxxxxxxxxxxxx
> > > ---
> > >  drivers/pci/msi.c | 3 +++
> > >  1 file changed, 3 insertions(+)
> > > 
> > > diff --git a/drivers/pci/msi.c b/drivers/pci/msi.c
> > > index e5e75331b415..3a9f4f8ad8f9 100644
> > > --- a/drivers/pci/msi.c
> > > +++ b/drivers/pci/msi.c
> > > @@ -776,6 +776,9 @@ static void msix_mask_all(void __iomem *base, int 
> > > tsize)
> > >   u32 ctrl = PCI_MSIX_ENTRY_CTRL_MASKBIT;
> > >   int i;
> > >  
> > > + if (pci_msi_ignore_mask)
> > > +         return;
> > > +
> > >   for (i = 0; i < tsize; i++, base += PCI_MSIX_ENTRY_SIZE)
> > >           writel(ctrl, base + PCI_MSIX_ENTRY_VECTOR_CTRL);
> > >  }
> > > -- 
> > > 2.31.1
> > > 
> 
> -- 
> Best Regards,
> Marek Marczykowski-Górecki
> Invisible Things Lab