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SubjectRe: [PATCH [RFC]] PCI: hv: add explicit fencing to config space access
On Tue, May 03, 2016 at 02:22:00PM +0200, Vitaly Kuznetsov wrote:
> I'm trying to pass-through Broadcom BCM5720 NIC (Dell Device 1f5b) on Dell
> R720 server. Everything works fine when target VM has only one CPU, but
> SMP guests reboot when NIC driver is trying to access PCI config space
> (with hv_pcifront_read_config/hv_pcifront_write_config). The reboot
> appears to be induced by the hypervisor and no crash is observed. Windows
> event logs are not helpful at all ('Virtual machine ... has quit
> unexpectedly'). The particular access point is always different and
> putting debug between them (printk/mdelay/...) moves the issue further
> away. The server model affects the issue as well: on Dell R420 I'm able to
> pass-through BCM5720 NIC to SMP guests without issues.
>
> While I'm obviously failing to reveal the essence of the issue I was able
> to come up with a (possible) solution: if explicit fencing is put to
> hv_pcifront_read_config/hv_pcifront_write_config the issue goes away. The
> essential minimum is rmb() at the end on _hv_pcifront_read_config() and
> wmb() at the end of _hv_pcifront_write_config() but I'm not confident it
> will be sufficient for all hardware. I suggest the following fencing:
> 1) wmb()/mb() between choosing the function and writing to its space.
> 2) mb() before releasing the spinlock in both _hv_pcifront_read_config()/
> _hv_pcifront_write_config to ensure that consecutive reads/writes to
> the space won't get re-ordered as drivers may count on that.
> Config space access is not supposed to be performance-critical so this
> explicit fencing is not supposed to bring any slowdown.
>
> Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>

Applied with Jake's ack to pci/host-hv for v4.7, thanks, Vitaly.

I changed "fence" to "barrier" in the changelog to follow the
common Linux terminology.

> ---
> drivers/pci/host/pci-hyperv.c | 14 ++++++++++++++
> 1 file changed, 14 insertions(+)
>
> diff --git a/drivers/pci/host/pci-hyperv.c b/drivers/pci/host/pci-hyperv.c
> index c17e792..7e9b2de 100644
> --- a/drivers/pci/host/pci-hyperv.c
> +++ b/drivers/pci/host/pci-hyperv.c
> @@ -553,6 +553,8 @@ static void _hv_pcifront_read_config(struct hv_pci_dev *hpdev, int where,
> spin_lock_irqsave(&hpdev->hbus->config_lock, flags);
> /* Choose the function to be read. (See comment above) */
> writel(hpdev->desc.win_slot.slot, hpdev->hbus->cfg_addr);
> + /* Make sure the function was chosen before we start reading. */
> + mb();
> /* Read from that function's config space. */
> switch (size) {
> case 1:
> @@ -565,6 +567,11 @@ static void _hv_pcifront_read_config(struct hv_pci_dev *hpdev, int where,
> *val = readl(addr);
> break;
> }
> + /*
> + * Make sure the write was done before we release the spinlock
> + * allowing consecutive reads/writes.
> + */
> + mb();
> spin_unlock_irqrestore(&hpdev->hbus->config_lock, flags);
> } else {
> dev_err(&hpdev->hbus->hdev->device,
> @@ -592,6 +599,8 @@ static void _hv_pcifront_write_config(struct hv_pci_dev *hpdev, int where,
> spin_lock_irqsave(&hpdev->hbus->config_lock, flags);
> /* Choose the function to be written. (See comment above) */
> writel(hpdev->desc.win_slot.slot, hpdev->hbus->cfg_addr);
> + /* Make sure the function was chosen before we start writing. */
> + wmb();
> /* Write to that function's config space. */
> switch (size) {
> case 1:
> @@ -604,6 +613,11 @@ static void _hv_pcifront_write_config(struct hv_pci_dev *hpdev, int where,
> writel(val, addr);
> break;
> }
> + /*
> + * Make sure the write was done before we release the spinlock
> + * allowing consecutive reads/writes.
> + */
> + mb();
> spin_unlock_irqrestore(&hpdev->hbus->config_lock, flags);
> } else {
> dev_err(&hpdev->hbus->hdev->device,
> --
> 2.5.5
>

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