Subject: Re: [PATCH V5 1/6] powerpc/powernv: don't enable SRIOV when VF BAR has non 64bit-prefetchable BAR

From: Wei Yang <[email-masked]>
Date: Mon, 12 Oct 2015 10:58:37 +0800

On Fri, Oct 09, 2015 at 07:15:19PM +1100, Benjamin Herrenschmidt wrote:
>On Fri, 2015-10-09 at 10:46 +0800, Wei Yang wrote:
>> On PHB_IODA2, we enable SRIOV devices by mapping IOV BAR with M64 BARs. If
>> a SRIOV device's IOV BAR is not 64bit-prefetchable, this is not assigned
>> from 64bit prefetchable window, which means M64 BAR can't work on it.
>
>Won't this cause a lot of devices to become unsupported for us ? Or do
>all devices we care about have their BARs marked prefetchable ?
>

You are right. After doing so, some of the devices will not be supported.

Hmm, I thought you know this, since this strategy, use M64 BAR to map IOV
BAR, is proposed by you. This patch doesn't change the function, while make it
more explicit.

>> The reason is PCI bridges support only 2 windows and the kernel code
>> programs bridges in the way that one window is 32bit-nonprefetchable and
>> the other one is 64bit-prefetchable. So if devices' IOV BAR is 64bit and
>> non-prefetchable, it will be mapped into 32bit space and therefore M64
>> cannot be used for it.
>> 
>> This patch makes this explicit.
>
>So PCIe allows for non-prefetchable BARs to be put under prefetchable
>bridge windows as long as the mapping done by the CPU doesn't prefetch,
>I believe. Well it's a natural conclusion of the weird note "Additional
>Guidance on the Prefetchable Bit in Memory Space BARs" page 596 of PCIe
>spec v3.0... it also says that devices should be pretty much free to
>set their prefetchable bit even if they have side effects so.
>
>So maybe we should have that option, rather than just not using the
>devices, allow them to be allocate via the prefetchable window...
>

Based on current linux kernel pci core resource size/assignment algorithm,
when we have 64bit-prefetchable root window, only 64bit-prefetchable BAR could
be assigned in this region. Refer to commit 5b28541552ef "PCI: Restrict 64-bit
prefetchable bridge windows to 64-bit resources" for more detail.

So we have two facts here:
1. We want to use M64 BAR to map VFs, so that VF could be put into individual
PE. And the M64 BAR just map 64bit window.
2. Current MMIO size/assignment algorithm in linux pci core only put
64bit-prefetchable BAR into 64bit prefetchable bridge window, no other BARs.

Your suggestion here is to put a non-prefetchable BAR in prefetchable bridge
window? This would make the MMIO size/assignment algorithm more confusing.

>> Signed-off-by: Wei Yang <[email-masked]>
>> Reviewed-by: Gavin Shan <[email-masked]>
>> Acked-by: Alexey Kardashevskiy <[email-masked]>
>> ---
>>  arch/powerpc/platforms/powernv/pci-ioda.c | 25 +++++++++----------------
>>  1 file changed, 9 insertions(+), 16 deletions(-)
>> 
>> diff --git a/arch/powerpc/platforms/powernv/pci-ioda.c b/arch/powerpc/platforms/powernv/pci-ioda.c
>> index 85cbc96..8c031b5 100644
>> --- a/arch/powerpc/platforms/powernv/pci-ioda.c
>> +++ b/arch/powerpc/platforms/powernv/pci-ioda.c
>> @@ -908,9 +908,6 @@ static int pnv_pci_vf_resource_shift(struct pci_dev *dev, int offset)
>>  > 	> 	> if (!res->flags || !res->parent)
>>  > 	> 	> 	> continue;
>>  
>> -> 	> 	> if (!pnv_pci_is_mem_pref_64(res->flags))
>> -> 	> 	> 	> continue;
>> -
>>  > 	> 	> /*
>>  > 	> 	>  * The actual IOV BAR range is determined by the start address
>>  > 	> 	>  * and the actual size for num_vfs VFs BAR.  This check is to
>> @@ -939,9 +936,6 @@ static int pnv_pci_vf_resource_shift(struct pci_dev *dev, int offset)
>>  > 	> 	> if (!res->flags || !res->parent)
>>  > 	> 	> 	> continue;
>>  
>> -> 	> 	> if (!pnv_pci_is_mem_pref_64(res->flags))
>> -> 	> 	> 	> continue;
>> -
>>  > 	> 	> size = pci_iov_resource_size(dev, i + PCI_IOV_RESOURCES);
>>  > 	> 	> res2 = *res;
>>  > 	> 	> res->start += size * offset;
>> @@ -1221,9 +1215,6 @@ static int pnv_pci_vf_assign_m64(struct pci_dev *pdev, u16 num_vfs)
>>  > 	> 	> if (!res->flags || !res->parent)
>>  > 	> 	> 	> continue;
>>  
>> -> 	> 	> if (!pnv_pci_is_mem_pref_64(res->flags))
>> -> 	> 	> 	> continue;
>> -
>>  > 	> 	> for (j = 0; j < vf_groups; j++) {
>>  > 	> 	> 	> do {
>>  > 	> 	> 	> 	> win = find_next_zero_bit(&phb->ioda.m64_bar_alloc,
>> @@ -1510,6 +1501,12 @@ int pnv_pci_sriov_enable(struct pci_dev *pdev, u16 num_vfs)
>>  > 	> pdn = pci_get_pdn(pdev);
>>  
>>  > 	> if (phb->type == PNV_PHB_IODA2) {
>> +> 	> 	> if (!pdn->vfs_expanded) {
>> +> 	> 	> 	> dev_info(&pdev->dev, "don't support this SRIOV device"
>> +> 	> 	> 	> 	> " with non 64bit-prefetchable IOV BAR\n");
>> +> 	> 	> 	> return -ENOSPC;
>> +> 	> 	> }
>> +
>>  > 	> 	> /* Calculate available PE for required VFs */
>>  > 	> 	> mutex_lock(&phb->ioda.pe_alloc_mutex);
>>  > 	> 	> pdn->offset = bitmap_find_next_zero_area(
>> @@ -2775,9 +2772,10 @@ static void pnv_pci_ioda_fixup_iov_resources(struct pci_dev *pdev)
>>  > 	> 	> if (!res->flags || res->parent)
>>  > 	> 	> 	> continue;
>>  > 	> 	> if (!pnv_pci_is_mem_pref_64(res->flags)) {
>> -> 	> 	> 	> dev_warn(&pdev->dev, " non M64 VF BAR%d: %pR\n",
>> +> 	> 	> 	> dev_warn(&pdev->dev, "Don't support SR-IOV with"
>> +> 	> 	> 	> 	> 	> " non M64 VF BAR%d: %pR. \n",
>>  > 	> 	> 	> 	>  i, res);
>> -> 	> 	> 	> continue;
>> +> 	> 	> 	> return;
>>  > 	> 	> }
>>  
>>  > 	> 	> size = pci_iov_resource_size(pdev, i + PCI_IOV_RESOURCES);
>> @@ -2796,11 +2794,6 @@ static void pnv_pci_ioda_fixup_iov_resources(struct pci_dev *pdev)
>>  > 	> 	> res = &pdev->resource[i + PCI_IOV_RESOURCES];
>>  > 	> 	> if (!res->flags || res->parent)
>>  > 	> 	> 	> continue;
>> -> 	> 	> if (!pnv_pci_is_mem_pref_64(res->flags)) {
>> -> 	> 	> 	> dev_warn(&pdev->dev, "Skipping expanding VF BAR%d: %pR\n",
>> -> 	> 	> 	> 	>  i, res);
>> -> 	> 	> 	> continue;
>> -> 	> 	> }
>>  
>>  > 	> 	> dev_dbg(&pdev->dev, " Fixing VF BAR%d: %pR to\n", i, res);
>>  > 	> 	> size = pci_iov_resource_size(pdev, i + PCI_IOV_RESOURCES);

-- 
Richard Yang
Help you, Help me