linux-kernel - Re: [PATCH v2 2/2] vfio/pci: Don't regenerate vconfig for all BARs if !bardirty

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Message-ID: <8cc2acc1-6eaa-8ed0-db78-7ff5dc36828d@huawei.com>
Date:   Mon, 21 Sep 2020 19:28:25 +0800
From:   Zenghui Yu <yuzenghui@...wei.com>
To:     Cornelia Huck <cohuck@...hat.com>
CC:     <kvm@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <alex.williamson@...hat.com>, <wanghaibin.wang@...wei.com>
Subject: Re: [PATCH v2 2/2] vfio/pci: Don't regenerate vconfig for all BARs if
 !bardirty

Hi Cornelia,

On 2020/9/21 18:21, Cornelia Huck wrote:
> On Mon, 21 Sep 2020 12:51:16 +0800
> Zenghui Yu <yuzenghui@...wei.com> wrote:
> 
>> Now we regenerate vconfig for all the BARs via vfio_bar_fixup(), every time
>> any offset of any of them are read. Though BARs aren't re-read regularly,
>> the regeneration can be avoid if no BARs had been written since they were
> 
> s/avoid/avoided/
> 
>> last read, in which case the vdev->bardirty is false.
> 
> s/the//
> 
>>
>> Let's predicate the vfio_bar_fixup() on the bardirty so that it can return
>> immediately if !bardirty.
> 
> Maybe
> 
> "Let's return immediately in vfio_bar_fixup() if bardirty is false." ?

Yes.

>>
>> Suggested-by: Alex Williamson <alex.williamson@...hat.com>
>> Signed-off-by: Zenghui Yu <yuzenghui@...wei.com>
>> ---
>> * From v1:
>>    - Per Alex's suggestion, let vfio_bar_fixup() test vdev->bardirty to
>>      avoid doing work if bardirty is false, instead of removing it entirely.
>>    - Rewrite the commit message.
>>
>>   drivers/vfio/pci/vfio_pci_config.c | 3 +++
>>   1 file changed, 3 insertions(+)
>>
>> diff --git a/drivers/vfio/pci/vfio_pci_config.c b/drivers/vfio/pci/vfio_pci_config.c
>> index d98843feddce..5e02ba07e8e8 100644
>> --- a/drivers/vfio/pci/vfio_pci_config.c
>> +++ b/drivers/vfio/pci/vfio_pci_config.c
>> @@ -467,6 +467,9 @@ static void vfio_bar_fixup(struct vfio_pci_device *vdev)
>>   	__le32 *vbar;
>>   	u64 mask;
>>
>> +	if (!vdev->bardirty)
> 
> Finally, bardirty can actually affect something :)
> 
>> +		return;
>> +
>>   	vbar = (__le32 *)&vdev->vconfig[PCI_BASE_ADDRESS_0];
>>   
>>   	for (i = 0; i < PCI_STD_NUM_BARS; i++, vbar++) {
> 
> Reviewed-by: Cornelia Huck <cohuck@...hat.com>

Thanks for you review! I think Alex can help fix the commit message when
applying? Otherwise I can send a v3.


Thanks,
Zenghui