Re: [PATCH] vpci: Add resizable bar support

["Chen, Jiqian" <Jiqian.Chen@xxxxxxx>]

On 2024/11/19 20:46, Roger Pau Monné wrote:
> On Mon, Nov 18, 2024 at 06:06:03AM +0000, Chen, Jiqian wrote:
>> On 2024/11/15 19:42, Roger Pau Monné wrote:
>>> On Fri, Nov 15, 2024 at 03:04:22AM +0000, Chen, Jiqian wrote:
>>>> On 2024/11/15 01:36, Roger Pau Monné wrote:
>>>>> On Thu, Nov 14, 2024 at 04:52:18PM +0100, Roger Pau Monné wrote:
>>>>>> On Thu, Nov 14, 2024 at 06:11:46AM +0000, Chen, Jiqian wrote:
>>>>>>> On 2024/11/13 18:30, Roger Pau Monné wrote:
>>>>>>>> On Wed, Nov 13, 2024 at 10:00:33AM +0000, Chen, Jiqian wrote:
>>>>>>>>> On 2024/11/13 17:30, Roger Pau Monné wrote:
>>>>>>>>>> On Wed, Nov 13, 2024 at 04:00:27PM +0800, Jiqian Chen wrote:
>>>>>>>>>>> Some devices, like discrete GPU of amd, support resizable bar 
>>>>>>>>>>> capability,
>>>>>>>>>>> but vpci of Xen doesn't support this feature, so they fail to 
>>>>>>>>>>> resize bars
>>>>>>>>>>> and then cause probing failure.
>>>>>>>>>>>
>>>>>>>>>>> According to PCIe spec, each bar that support resizing has two 
>>>>>>>>>>> registers,
>>>>>>>>>>> PCI_REBAR_CAP and PCI_REBAR_CTRL, so add these two registers and 
>>>>>>>>>>> their
>>>>>>>>>>> corresponding handler into vpci.
>>>>>>>>>>>
>>>>>>>>>>> PCI_REBAR_CAP is RO, only provide reading.
>>>>>>>>>>>
>>>>>>>>>>> PCI_REBAR_CTRL only has bar size is RW, so add write function to 
>>>>>>>>>>> support
>>>>>>>>>>> setting the new size.
>>>>>>>>>>
>>>>>>>>>> I think the logic to handle resizable BAR could be much simpler.  
>>>>>>>>>> Some
>>>>>>>>>> time ago I've made a patch to add support for it, but due to lack of
>>>>>>>>>> hardware on my side to test it I've never submitted it.
>>>>>>>>>>
>>>>>>>>>> My approach would be to detect the presence of the
>>>>>>>>>> PCI_EXT_CAP_ID_REBAR capability in init_header(), and if the
>>>>>>>>>> capability is present force the sizing of BARs each time they are
>>>>>>>>>> mapped in modify_bars().  I don't think we need to trap accesses to
>>>>>>>>>> the capability itself, as resizing can only happen when memory
>>>>>>>>>> decoding is not enabled for the device.  It's enough to fetch the 
>>>>>>>>>> size
>>>>>>>>>> of the BARs ahead of each enabling of memory decoding.
>>>>>>>>>>
>>>>>>>>>> Note that memory decoding implies mapping the BARs into the p2m, 
>>>>>>>>>> which
>>>>>>>>>> is already an expensive operation, the extra sizing is unlikely to
>>>>>>>>>> make much of a difference performance wise.
>>>>>>>>>>
>>>>>>>>>> I've found the following on my git tree and rebased on top of 
>>>>>>>>>> staging:
>>>>>>>>> OK.
>>>>>>>>> Do you need me to validate your patch in my environment?
>>>>>>>>
>>>>>>>> Yes please, I have no way to test it.  Let's see what others think
>>>>>>>> about the different approaches.
>>>>>>> There are some errors with your method.
>>>>>>> I attached the dmesg and xl dmesg logs.
>>>>>>> From the dmesg logs, it seems that 0000:03:00.0 has addresse overlap 
>>>>>>> with 0000:03:00.1
>>>>>>
>>>>>> Do you have the output of lspci with the BAR sizes/positions before
>>>>>> and after the resizing?
>>>>>>
>>>>>>>
>>>>>>> I think there is a place that needs to be modified regarding your 
>>>>>>> method,
>>>>>>> although this modification does not help with the above-mentioned 
>>>>>>> errors,
>>>>>>> it is that whether to support resizing is specific to which bar, rather 
>>>>>>> than just determining whether there is a Rebar capability.
>>>>>>
>>>>>> Do we really need such fine-grained information?  It should be
>>>>>> harmless (even if not strictly necessary) to size all BARs on the
>>>>>> device before enabling memory decoding, even if some of them do not
>>>>>> support resizing.
>>>>>>
>>>>>> I might have to provide a patch with additional messages to see what's
>>>>>> going on.
>>>>>
>>>>> One nit that I've noticed with the patch I gave you previously is that
>>>>> the check for a size change in modify_bars() should be done ahead of
>>>>> pci_check_bar(), otherwise the check is possibly using an outdated
>>>>> size.
>>>>>
>>>>> I've also added a debug message to notify when a BAR register is
>>>>> written and report the new address.  This is done unconditionally, but
>>>>> if you think it's too chatty you can limit to only printing for the
>>>>> device that has the ReBAR capability.
>>>> Errors are the same.
>>>> Attached the dmesg, xl dmesg, patch and lspci output.
>>>> I will also continue to debug your method on my side to try to get some 
>>>> findings.
>>>
>>> Hello,
>>>
>>> I've been looking at the output, and it all seems fine, except the
>>> 03:00.0 device that becomes broken at some point, note the lspci
>>> output lists [virtual] next to the resource sizes.  This means reading
>>> for the registers returned 0, so the position and sizes are provided
>>> from the internal OS information.
>>>
>>> I'm assuming the patch you sent to the list doesn't lead to such errors,
>> Yes, the method of my patch doesn't lead to any errors.
>> I attached the dmesg, xl dmesg and lspci logs of my method.
>>
>>> in which case I can only guess that fetching the size of the
>>> BARs in modify_bars() causes issues with the device.
>>>
>>> To confirm this, can you try the following patch on top of your original 
>>> change?  
>> I tried below patch with my original patch, it didn't cause any errors.
>> And the lspci showed without the "[virtual]".
>> So, unfortunately, it is not related to the fetching size of Bars in 
>> modify_bars().
> 
> I see, I'm at a loss as to what's wrong with my patch.  Do you have
> any additional patches on Xen when testing your or my approach?
No, just my patch or your patch, base on upstream codes(kernel branch is 
linux-next and xen branch is staging).

> 
> I sadly don't have any box with a PCI device that exposes the
> resizable BAR capability, so I'm not able to test or debug this.
> 
> I would like to understand why my approach doesn't work, as otherwise
> I feel like I'm missing how ReBAR is supposed to work.  Anyway, if you
> can give a try to the diff below, it's the same patch, but with yet
> even more debug messages added.
Attached the xl dmesg.

> 
> Thanks, and sorry for keeping you testing it.
That's fine, feel free to send me if you need more test information.
Actually I am still continuing to debug your patch and also curious about why 
your patch does not work.

The only difference between our methods is the timing of updating the size.
Yours is later than mine because you updated the size when the driver 
re-enabled memory decoding, while I updated the size in time when driver resize 
it.
I suspect that the driver or hypervisor may have done something in between and 
read outdated information.
I am debugging the driver codes.

> 
> Regards, Roger.
> 
> diff --git a/xen/drivers/vpci/header.c b/xen/drivers/vpci/header.c
> index ef6c965c081c..dda42ef0b7ff 100644
> --- a/xen/drivers/vpci/header.c
> +++ b/xen/drivers/vpci/header.c
> @@ -316,6 +316,9 @@ static int modify_bars(const struct pci_dev *pdev, 
> uint16_t cmd, bool rom_only)
>  
>      ASSERT(rw_is_write_locked(&pdev->domain->pci_lock));
>  
> +    printk("%pp: modify bars cmd: %x rom_only: %d\n",
> +           &pdev->sbdf, cmd, rom_only);
> +
>      /*
>       * Create a rangeset per BAR that represents the current device memory
>       * region and compare it against all the currently active BAR memory
> @@ -346,6 +349,33 @@ static int modify_bars(const struct pci_dev *pdev, 
> uint16_t cmd, bool rom_only)
>               bar->enabled == !!(cmd & PCI_COMMAND_MEMORY) )
>              continue;
>  
> +        if ( bar->type != VPCI_BAR_ROM && header->bars_resizable &&
> +             (cmd & PCI_COMMAND_MEMORY) )
> +        {
> +            uint64_t addr, size;
> +
> +            pci_size_mem_bar(pdev->sbdf, PCI_BASE_ADDRESS_0 + i * 4,
> +                             &addr, &size, 0);
> +
> +            printk("%pp: BAR%u ReBAR supported addr %#lx -> %#lx size %#lx 
> -> %#lx\n",
> +                    &pdev->sbdf, i, bar->addr, addr, bar->size, size);
> +
> +            if ( bar->addr != addr )
> +                printk(XENLOG_G_ERR
> +                       "%pp: BAR#%u address mismatch %#lx vs %#lx\n",
> +                       &pdev->sbdf, i, bar->addr, addr);
> +
> +            if ( bar->size != size )
> +            {
> +                printk(XENLOG_G_DEBUG
> +                       "%pp: detected BAR#%u size change (%#lx -> %#lx)\n",
> +                       &pdev->sbdf, i, bar->size, size);
> +                bar->size = size;
> +                end = PFN_DOWN(bar->addr + size - 1);
> +                end_guest = PFN_DOWN(bar->guest_addr + size - 1);
> +            }
> +        }
> +
>          if ( !pci_check_bar(pdev, _mfn(start), _mfn(end)) )
>          {
>              printk(XENLOG_G_WARNING
> @@ -583,8 +613,6 @@ static void cf_check bar_write(
>       */
>      if ( bar->enabled )
>      {
> -        /* If the value written is the current one avoid printing a warning. 
> */
> -        if ( val != (uint32_t)(bar->addr >> (hi ? 32 : 0)) )
>              gprintk(XENLOG_WARNING,
>                      "%pp: ignored BAR %zu write while mapped\n",
>                      &pdev->sbdf, bar - pdev->vpci->header.bars + hi);
> @@ -601,6 +629,10 @@ static void cf_check bar_write(
>      /* Update guest address, so hardware domain BAR is identity mapped. */
>      bar->guest_addr = bar->addr;
>  
> +    printk("%pp: write BAR%zu val: %#x BAR%zu address: %#lx\n",
> +           &pdev->sbdf, bar - pdev->vpci->header.bars, val,
> +           bar - pdev->vpci->header.bars + hi, bar->addr);
> +
>      /* Make sure Xen writes back the same value for the BAR RO bits. */
>      if ( !hi )
>      {
> @@ -870,6 +902,9 @@ static int cf_check init_header(struct pci_dev *pdev)
>      if ( pdev->ignore_bars )
>          return 0;
>  
> +    header->bars_resizable = pci_find_ext_capability(pdev->sbdf,
> +                                                     PCI_EXT_CAP_ID_REBAR);
> +
>      cmd = pci_conf_read16(pdev->sbdf, PCI_COMMAND);
>  
>      /*
> diff --git a/xen/include/xen/pci_regs.h b/xen/include/xen/pci_regs.h
> index 250ba106dbd3..c543a2b86778 100644
> --- a/xen/include/xen/pci_regs.h
> +++ b/xen/include/xen/pci_regs.h
> @@ -459,6 +459,7 @@
>  #define PCI_EXT_CAP_ID_ARI   14
>  #define PCI_EXT_CAP_ID_ATS   15
>  #define PCI_EXT_CAP_ID_SRIOV 16
> +#define PCI_EXT_CAP_ID_REBAR 21
>  
>  /* Advanced Error Reporting */
>  #define PCI_ERR_UNCOR_STATUS 4       /* Uncorrectable Error Status */
> diff --git a/xen/include/xen/vpci.h b/xen/include/xen/vpci.h
> index 41e7c3bc2791..45ebc1bb3356 100644
> --- a/xen/include/xen/vpci.h
> +++ b/xen/include/xen/vpci.h
> @@ -129,6 +129,8 @@ struct vpci {
>           * upon to know whether BARs are mapped into the guest p2m.
>           */
>          bool bars_mapped      : 1;
> +        /* Device has the Resizable BARs capability. */
> +        bool bars_resizable   : 1;
>          /* FIXME: currently there's no support for SR-IOV. */
>      } header;
>  

-- 
Best regards,
Jiqian Chen.

Attachment: rebar_xl_dmesg.txt
Description: rebar_xl_dmesg.txt