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

From: Bjorn Helgaas <helgaas@kernel.org>
To: "Marek Marczykowski-Górecki" <marmarek@invisiblethingslab.com>
Cc: linux-kernel@vger.kernel.org, stable@vger.kernel.org,
	xen-devel@lists.xenproject.org,
	Bjorn Helgaas <bhelgaas@google.com>,
	Marc Zyngier <maz@kernel.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	"open list:PCI SUBSYSTEM" <linux-pci@vger.kernel.org>
Subject: Re: [PATCH] PCI/MSI: skip masking MSI on Xen PV
Date: Thu, 26 Aug 2021 09:55:32 -0500	[thread overview]
Message-ID: <20210826145532.GA3673811@bjorn-Precision-5520> (raw)
In-Reply-To: <20210826134337.134767-1-marmarek@invisiblethingslab.com>

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.

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@vger.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.

> Signed-off-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>
> ---
> Cc: xen-devel@lists.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
> 