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

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Date:   Thu, 26 Aug 2021 18:36:49 +0200
From:   Marek Marczykowski-Górecki 
        <marmarek@...isiblethingslab.com>
To:     Bjorn Helgaas <helgaas@...nel.org>
Cc:     linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        xen-devel@...ts.xenproject.org,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Marc Zyngier <maz@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "open list:PCI SUBSYSTEM" <linux-pci@...r.kernel.org>
Subject: Re: [PATCH] PCI/MSI: skip masking MSI on Xen PV

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?

> 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@...r.kernel.org
> 
> 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...

> > Signed-off-by: Marek Marczykowski-Górecki <marmarek@...isiblethingslab.com>
> > ---
> > Cc: xen-devel@...ts.xenproject.org
> > ---
> >  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

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