[v4,5/5] PCI: Add a macro to set default alignment for all PCI devices

When vfio passthroughs a PCI device of which MMIO BARs are
smaller than PAGE_SIZE, guest will not handle the mmio
accesses to the BARs which leads to mmio emulations in host.

This is because vfio will not allow to passthrough one BAR's
mmio page which may be shared with other BARs. Otherwise,
there will be a backdoor that guest can use to access BARs
of other guest.

This patch adds a macro to set default alignment for all
PCI devices. Then we could solve this issue on some platforms
which would easily hit this issue because of their 64K page
such as PowerNV platform by defining this macro as PAGE_SIZE.

Signed-off-by: Yongji Xie <xyjxie@linux.vnet.ibm.com>
---
 arch/powerpc/include/asm/pci.h |    4 ++++
 drivers/pci/pci.c              |    4 ++++
 2 files changed, 8 insertions(+)

On Fri, Aug 12, 2016 at 01:42:26PM +0800, Yongji Xie wrote:
> When vfio passthroughs a PCI device of which MMIO BARs are
> smaller than PAGE_SIZE, guest will not handle the mmio
> accesses to the BARs which leads to mmio emulations in host.
> 
> This is because vfio will not allow to passthrough one BAR's
> mmio page which may be shared with other BARs. Otherwise,
> there will be a backdoor that guest can use to access BARs
> of other guest.
> 
> This patch adds a macro to set default alignment for all
> PCI devices. Then we could solve this issue on some platforms
> which would easily hit this issue because of their 64K page
> such as PowerNV platform by defining this macro as PAGE_SIZE.

Just to clarify, I think the issue happens on any arch, whenever
device BARs are smaller than PAGE_SIZE.  This is obviously more
*likely* when PAGE_SIZE is large, but could still happen even with
4K pages.

> Signed-off-by: Yongji Xie <xyjxie@linux.vnet.ibm.com>
> ---
>  arch/powerpc/include/asm/pci.h |    4 ++++
>  drivers/pci/pci.c              |    4 ++++
>  2 files changed, 8 insertions(+)
> 
> diff --git a/arch/powerpc/include/asm/pci.h b/arch/powerpc/include/asm/pci.h
> index e9bd6cf..5e31bc2 100644
> --- a/arch/powerpc/include/asm/pci.h
> +++ b/arch/powerpc/include/asm/pci.h
> @@ -28,6 +28,10 @@
>  #define PCIBIOS_MIN_IO		0x1000
>  #define PCIBIOS_MIN_MEM		0x10000000
>  
> +#ifdef CONFIG_PPC_POWERNV
> +#define PCIBIOS_DEFAULT_ALIGNMENT	PAGE_SIZE
> +#endif
> +
>  struct pci_dev;
>  
>  /* Values for the `which' argument to sys_pciconfig_iobase syscall.  */
> diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
> index d895be7..feae59e 100644
> --- a/drivers/pci/pci.c
> +++ b/drivers/pci/pci.c
> @@ -4959,6 +4959,10 @@ static resource_size_t pci_specified_resource_alignment(struct pci_dev *dev,
>  	resource_size_t align = 0;
>  	char *p;
>  
> +#ifdef PCIBIOS_DEFAULT_ALIGNMENT
> +	align = PCIBIOS_DEFAULT_ALIGNMENT;
> +	*resize = false;
> +#endif
>  	spin_lock(&resource_alignment_lock);
>  	p = resource_alignment_param;
>  	if (pci_has_flag(PCI_PROBE_ONLY)) {
> -- 
> 1.7.9.5
> 
> --
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On 2016/9/7 0:59, Bjorn Helgaas wrote:

> On Fri, Aug 12, 2016 at 01:42:26PM +0800, Yongji Xie wrote:
>> When vfio passthroughs a PCI device of which MMIO BARs are
>> smaller than PAGE_SIZE, guest will not handle the mmio
>> accesses to the BARs which leads to mmio emulations in host.
>>
>> This is because vfio will not allow to passthrough one BAR's
>> mmio page which may be shared with other BARs. Otherwise,
>> there will be a backdoor that guest can use to access BARs
>> of other guest.
>>
>> This patch adds a macro to set default alignment for all
>> PCI devices. Then we could solve this issue on some platforms
>> which would easily hit this issue because of their 64K page
>> such as PowerNV platform by defining this macro as PAGE_SIZE.
> Just to clarify, I think the issue happens on any arch, whenever
> device BARs are smaller than PAGE_SIZE.  This is obviously more
> *likely* when PAGE_SIZE is large, but could still happen even with
> 4K pages.

Yes, that's true.

Thanks,
Yongji

>> Signed-off-by: Yongji Xie <xyjxie@linux.vnet.ibm.com>
>> ---
>>   arch/powerpc/include/asm/pci.h |    4 ++++
>>   drivers/pci/pci.c              |    4 ++++
>>   2 files changed, 8 insertions(+)
>>
>> diff --git a/arch/powerpc/include/asm/pci.h b/arch/powerpc/include/asm/pci.h
>> index e9bd6cf..5e31bc2 100644
>> --- a/arch/powerpc/include/asm/pci.h
>> +++ b/arch/powerpc/include/asm/pci.h
>> @@ -28,6 +28,10 @@
>>   #define PCIBIOS_MIN_IO		0x1000
>>   #define PCIBIOS_MIN_MEM		0x10000000
>>   
>> +#ifdef CONFIG_PPC_POWERNV
>> +#define PCIBIOS_DEFAULT_ALIGNMENT	PAGE_SIZE
>> +#endif
>> +
>>   struct pci_dev;
>>   
>>   /* Values for the `which' argument to sys_pciconfig_iobase syscall.  */
>> diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
>> index d895be7..feae59e 100644
>> --- a/drivers/pci/pci.c
>> +++ b/drivers/pci/pci.c
>> @@ -4959,6 +4959,10 @@ static resource_size_t pci_specified_resource_alignment(struct pci_dev *dev,
>>   	resource_size_t align = 0;
>>   	char *p;
>>   
>> +#ifdef PCIBIOS_DEFAULT_ALIGNMENT
>> +	align = PCIBIOS_DEFAULT_ALIGNMENT;
>> +	*resize = false;
>> +#endif
>>   	spin_lock(&resource_alignment_lock);
>>   	p = resource_alignment_param;
>>   	if (pci_has_flag(PCI_PROBE_ONLY)) {
>> -- 
>> 1.7.9.5
>>
>> --
>> To unsubscribe from this list: send the line "unsubscribe linux-pci" in
>> the body of a message to majordomo@vger.kernel.org
>> More majordomo info at  http://vger.kernel.org/majordomo-info.html