From bdd74440d9e887b1fa648eefa17421def5f5243c Mon Sep 17 00:00:00 2001 From: Vitaly Kuznetsov Date: Tue, 3 May 2016 14:22:00 +0200 Subject: [PATCH] PCI: hv: Add explicit barriers to config space access I'm trying to pass-through Broadcom BCM5720 NIC (Dell device 1f5b) on a Dell R720 server. Everything works fine when the target VM has only one CPU, but SMP guests reboot when the NIC driver accesses 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 barriers are added 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 barriers: 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 these explicit barriers should not cause any slowdown. [bhelgaas: use Linux "barriers" terminology] Signed-off-by: Vitaly Kuznetsov Signed-off-by: Bjorn Helgaas Acked-by: Jake Oshins --- 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 328c653d7503..58f7eeb79bb4 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, -- 2.39.5