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Message-ID: <6827AF44-4412-4ABA-A66B-562D7E86C847@vmware.com>
Date: Thu, 3 Feb 2022 05:52:30 +0000
From: Vikash Bansal <bvikas@...are.com>
To: Bjorn Helgaas <helgaas@...nel.org>
CC: "bhelgaas@...gle.com" <bhelgaas@...gle.com>,
"linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
"gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
Srivatsa Bhat <srivatsab@...are.com>,
"srivatsa@...il.mit.edu" <srivatsa@...il.mit.edu>,
Alexey Makhalov <amakhalov@...are.com>,
Srinidhi Rao <srinidhir@...are.com>,
Anish Swaminathan <anishs@...are.com>,
Vasavi Sirnapalli <vsirnapalli@...are.com>,
Ajay Kaher <akaher@...are.com>,
"ymcavikas@...il.com" <ymcavikas@...il.com>
Subject: Re: [PATCH v2] PCI: Speed up device init by parsing capabilities all
at once
On 03/02/22, 3:05 AM, "Bjorn Helgaas" <helgaas@...nel.org> wrote:
> On Fri, Jan 28, 2022 at 04:51:43AM +0000, Vikash Bansal wrote:
Note: I am switching employer so from tomorrow onwards my VMWare ID
"bvikas@...are.com" will not be reachable. I will be reachable on my
Personal id "ymcavikas@...il.com". You may see new patches on this thread
From one of my colleague from in VMWare.
More comments below.
>> In the current implementation, the PCI capability list is parsed from
>> the beginning to find each capability, which results in a large number
>> of redundant PCI reads.
>>
>> Instead, we can parse the complete list just once, store it in the
>> pci_dev structure, and get the offset of each capability directly from
>> the pci_dev structure.
>>
>> This implementation improves pci devices initialization time by ~2-3%
>> (from 270ms to 261ms) in case of bare metal and 7-8% (From 201ms to 184ms)
>> in case of VM running on ESXi.
>>
>> It also adds a memory overhead of 20bytes (value of PCI_CAP_ID_MAX) per
>> PCI device.
>>
>> Ran pahole for pci_dev structure. This patch is not adding any padding
>> bytes.
>
> I like the idea of this.
>
> I guess the corresponding thing could be done for extended
> capabilities as well, but they're not as common, there are more of
> them, and maybe the benefit wouldn't be worth it.
>
> More comments below.
>
You are right, this thing can be done for extended capability.
We can have similar patch for extended capability also in future.
More comment below.
>> Signed-off-by: Vikash Bansal <bvikas@...are.com>
>> ---
>>
>> Changes in v2:
>> - Ran pahole tool.
>> - Modified comments to add "clock time".
>> - Removed comments before call to pci_find_all_capabilities.
>>
>> drivers/pci/pci.c | 43 ++++++++++++++++++++++++++++++++++++-------
>> drivers/pci/probe.c | 1 +
>> include/linux/pci.h | 2 ++
>> 3 files changed, 39 insertions(+), 7 deletions(-)
>>
>> diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
>> index 9ecce435fb3f..b361788bcc27 100644
>> --- a/drivers/pci/pci.c
>> +++ b/drivers/pci/pci.c
>> @@ -468,6 +468,41 @@ static u8 __pci_bus_find_cap_start(struct pci_bus *bus,
>> return 0;
>> }
>>
>> +
>> +/**
>> + * pci_find_all_capabilities - Read all capabilities
>> + * @dev: the PCI device
>> + *
>> + * Read all capabilities and store offsets in cap_off
>> + * array in pci_dev structure.
>> + */
>> +void pci_find_all_capabilities(struct pci_dev *dev)
>> +{
>> + int ttl = PCI_FIND_CAP_TTL;
>> + u16 ent;
>> + u8 pos;
>> + u8 id;
>> +
>> + pos = __pci_bus_find_cap_start(dev->bus, dev->devfn, dev->hdr_type);
>> + if (!pos)
>> + return;
>> + pci_bus_read_config_byte(dev->bus, dev->devfn, pos, &pos);
>> + while (ttl--) {
>> + if (pos < 0x40)
>> + break;
>> + pos &= ~3;
>> + pci_bus_read_config_word(dev->bus, dev->devfn, pos, &ent);
>> + id = ent & 0xff;
>> + if (id == 0xff)
>> + break;
>> +
>> + /* Read first instance of capability */
>> + if (!(dev->cap_off[id]))
>> + dev->cap_off[id] = pos;
>> + pos = (ent >> 8);
>> + }
>> +}
>
> With the current capability infrastructure, one has to specify the
> capability of interest up front, so there's no way to iterate directly
> through all the capabilities. Hence this code that looks a lot like
> __pci_find_next_cap_ttl(), which knows the capability to look for and
> ignores any others.
>
Yes, I have taken login for pci_find_all_capabilities from "__pci_find_next_cap_tt ".
More comments below.
>> +
>> /**
>> * pci_find_capability - query for devices' capabilities
>> * @dev: PCI device to query
>> @@ -489,13 +524,7 @@ static u8 __pci_bus_find_cap_start(struct pci_bus *bus,
>> */
>> u8 pci_find_capability(struct pci_dev *dev, int cap)
>> {
>> - u8 pos;
>> -
>> - pos = __pci_bus_find_cap_start(dev->bus, dev->devfn, dev->hdr_type);
>> - if (pos)
>> - pos = __pci_find_next_cap(dev->bus, dev->devfn, pos, cap);
>> -
>> - return pos;
>> + return dev->cap_off[cap];
>
> This may index past the end of the cap_off[] table. We have no
> Control over the callers, so no way to prevent them from reading
> arbitrary memory here.
>
We can add below condition to prevent this
u8 pci_find_capability(struct pci_dev *dev, int cap)
{
+ if(cap > PCI_CAP_ID_MAX)
+ return 0;
return dev->cap_off[cap];
}
More comments below.
> Here's an alternate proposal:
>
> - Leave pci_find_capability() completely alone.
>
> - Don't add the cap_off[] table. This only works for Capabilities
> (not Extended Capabilities), depends on PCI_CAP_ID_MAX (which is
> not fixed and could change in the future), and duplicates existing
> data like dev->pcie_cap, dev->msi_cap, dev->msix_cap, etc.
>
> - Add pci_find_all_capabilities() (I'd probably call it
> "pci_scan_capabilities()"), but extend it to iterate through both
> kinds of capabilities (probably a helper function for each kind)
> and fill in dev->pcie_cap, dev->msi_cap, dev->aer_cap, etc as it
> finds them.
>
> - Follow up with patches that remove calls to pci_find_capability()
> and pci_find_ext_capability() for the capabilities we've already
> located.
>
I thought of having solution more generic. Using dev->pcie_cap,
dev->msi_cap, dev->aer_cap will take care of only 3 capabilities.
cap_off[] array will handle all equal to PCI_CAP_ID_MAX.
More comments below.
>> }
>> EXPORT_SYMBOL(pci_find_capability);
>>
>> diff --git a/drivers/pci/probe.c b/drivers/pci/probe.c
>> index 17a969942d37..b2fa5b2c42f6 100644
>> --- a/drivers/pci/probe.c
>> +++ b/drivers/pci/probe.c
>> @@ -1831,6 +1831,7 @@ int pci_setup_device(struct pci_dev *dev)
>> dev->hdr_type = hdr_type & 0x7f;
>> dev->multifunction = !!(hdr_type & 0x80);
>> dev->error_state = pci_channel_io_normal;
>> + pci_find_all_capabilities(dev);
>> set_pcie_port_type(dev);
>>
>> pci_set_of_node(dev);
>> diff --git a/include/linux/pci.h b/include/linux/pci.h
>> index 8253a5413d7c..abcf7fdc4c98 100644
>> --- a/include/linux/pci.h
>> +++ b/include/linux/pci.h
>> @@ -335,6 +335,7 @@ struct pci_dev {
>> unsigned int class; /* 3 bytes: (base,sub,prog-if) */
>> u8 revision; /* PCI revision, low byte of class word */
>> u8 hdr_type; /* PCI header type (`multi' flag masked out) */
>> + u8 cap_off[PCI_CAP_ID_MAX]; /* Offsets of all pci capabilities */
>> #ifdef CONFIG_PCIEAER
>> u16 aer_cap; /* AER capability offset */
>> struct aer_stats *aer_stats; /* AER stats for this device */
>> @@ -1140,6 +1141,7 @@ void pci_sort_breadthfirst(void);
>>
>> u8 pci_bus_find_capability(struct pci_bus *bus, unsigned int devfn, int cap);
>> u8 pci_find_capability(struct pci_dev *dev, int cap);
>> +void pci_find_all_capabilities(struct pci_dev *dev);
>
> Shouldn't be exposed outside drivers/pci/; could move to
> drivers/pci/pci.h.
>
Will change this.
>> u8 pci_find_next_capability(struct pci_dev *dev, u8 pos, int cap);
>> u8 pci_find_ht_capability(struct pci_dev *dev, int ht_cap);
>> u8 pci_find_next_ht_capability(struct pci_dev *dev, u8 pos, int ht_cap);
>> --
>> 2.30.0
>>
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