Re: [PATCH] [RFC] vpci: allow BAR write while mapped

["Alejandro Vallejo" <alejandro.vallejo@xxxxxxxxx>]

On Wed Mar 12, 2025 at 7:50 PM GMT, Stewart Hildebrand wrote:
> Xen vPCI refuses BAR writes if the BAR is mapped in p2m. If firmware
> initialized the BAR to a bad address, Linux will try to write a new
> address to the BAR without disabling memory decoding. Allow the write
> by updating p2m right away in the vPCI BAR write handler.
>
> Resolves: https://gitlab.com/xen-project/xen/-/issues/197
> Signed-off-by: Stewart Hildebrand <stewart.hildebrand@xxxxxxx>
> ---
> RFC: Currently the deferred mapping machinery supports only map or
>      unmap, not both. It might be better to rework the mapping machinery
>      to support unmap-then-map operations, but please let me know your
>      thoughts.
> RFC: This patch has not yet made an attempt to distinguish between
>      32-bit and 64-bit writes. It probably should.
> ---
>  xen/drivers/vpci/header.c | 65 +++++++++++++++++++++++++++++++--------
>  1 file changed, 53 insertions(+), 12 deletions(-)
>
> diff --git a/xen/drivers/vpci/header.c b/xen/drivers/vpci/header.c
> index ef6c965c081c..66adb2183cfe 100644
> --- a/xen/drivers/vpci/header.c
> +++ b/xen/drivers/vpci/header.c
> @@ -173,7 +173,7 @@ static void modify_decoding(const struct pci_dev *pdev, 
> uint16_t cmd,
>          ASSERT_UNREACHABLE();
>  }
>  
> -bool vpci_process_pending(struct vcpu *v)
> +static bool process_pending(struct vcpu *v, bool need_lock)
>  {
>      struct pci_dev *pdev = v->vpci.pdev;
>      struct vpci_header *header = NULL;
> @@ -182,12 +182,14 @@ bool vpci_process_pending(struct vcpu *v)
>      if ( !pdev )
>          return false;
>  
> -    read_lock(&v->domain->pci_lock);
> +    if ( need_lock )
> +        read_lock(&v->domain->pci_lock);
>  
>      if ( !pdev->vpci || (v->domain != pdev->domain) )
>      {
>          v->vpci.pdev = NULL;
> -        read_unlock(&v->domain->pci_lock);
> +        if ( need_lock )
> +            read_unlock(&v->domain->pci_lock);
>          return false;
>      }
>  
> @@ -209,17 +211,20 @@ bool vpci_process_pending(struct vcpu *v)
>  
>          if ( rc == -ERESTART )
>          {
> -            read_unlock(&v->domain->pci_lock);
> +            if ( need_lock )
> +                read_unlock(&v->domain->pci_lock);
>              return true;
>          }
>  
>          if ( rc )
>          {
> -            spin_lock(&pdev->vpci->lock);
> +            if ( need_lock )
> +                spin_lock(&pdev->vpci->lock);
>              /* Disable memory decoding unconditionally on failure. */
>              modify_decoding(pdev, v->vpci.cmd & ~PCI_COMMAND_MEMORY,
>                              false);
> -            spin_unlock(&pdev->vpci->lock);
> +            if ( need_lock )
> +                spin_unlock(&pdev->vpci->lock);
>  
>              /* Clean all the rangesets */
>              for ( i = 0; i < ARRAY_SIZE(header->bars); i++ )
> @@ -228,7 +233,8 @@ bool vpci_process_pending(struct vcpu *v)
>  
>              v->vpci.pdev = NULL;
>  
> -            read_unlock(&v->domain->pci_lock);
> +            if ( need_lock )
> +                read_unlock(&v->domain->pci_lock);
>  
>              if ( !is_hardware_domain(v->domain) )
>                  domain_crash(v->domain);
> @@ -238,15 +244,23 @@ bool vpci_process_pending(struct vcpu *v)
>      }
>      v->vpci.pdev = NULL;
>  
> -    spin_lock(&pdev->vpci->lock);
> +    if ( need_lock )
> +        spin_lock(&pdev->vpci->lock);
>      modify_decoding(pdev, v->vpci.cmd, v->vpci.rom_only);
> -    spin_unlock(&pdev->vpci->lock);
> +    if ( need_lock )
> +        spin_unlock(&pdev->vpci->lock);
>  
> -    read_unlock(&v->domain->pci_lock);
> +    if ( need_lock )
> +        read_unlock(&v->domain->pci_lock);
>  
>      return false;
>  }
>  
> +bool vpci_process_pending(struct vcpu *v)
> +{
> +    return process_pending(v, true);
> +}
> +
>  static int __init apply_map(struct domain *d, const struct pci_dev *pdev,
>                              uint16_t cmd)
>  {
> @@ -565,6 +579,8 @@ static void cf_check bar_write(
>  {
>      struct vpci_bar *bar = data;
>      bool hi = false;
> +    bool reenable = false;
> +    uint32_t cmd = 0;
>  
>      ASSERT(is_hardware_domain(pdev->domain));
>  
> @@ -585,10 +601,31 @@ static void cf_check bar_write(
>      {
>          /* 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",
> +                    "%pp: allowing BAR %zu write while mapped\n",
>                      &pdev->sbdf, bar - pdev->vpci->header.bars + hi);
> -        return;
> +            ASSERT(rw_is_write_locked(&pdev->domain->pci_lock));
> +            ASSERT(spin_is_locked(&pdev->vpci->lock));
> +            reenable = true;
> +            cmd = pci_conf_read16(pdev->sbdf, PCI_COMMAND);
> +            /*
> +             * Write-while-mapped: unmap the old BAR in p2m. We want this to
> +             * finish right away since the deferral machinery only supports
> +             * unmap OR map, not unmap-then-remap. Ultimately, it probably 
> would
> +             * be better to defer the write-while-mapped case just like 
> regular
> +             * BAR writes (but still only allow it for 32-bit BAR writes).
> +             */
> +            /* Disable memory decoding */
> +            modify_bars(pdev, cmd & ~PCI_COMMAND_MEMORY, false);
> +            /* Call process pending here to ensure P2M operations are done */
> +            while ( process_pending(current, false) )
> +            {
> +                /* Pre-empted, try again */

This seems a tad dangerous. There may be a non-negligible amount of work queued
up. I also wonder whether the guest can induce spinning by increasing
contention on the p2m (e.g: via ballooning) or by induces work being queued up.

I don't quite understand the logic, but I suspect you could
raise_softirq(SCHEDULE_SOFTIRQ), decrease the IP so the instruction is
replayed, release the locks, and simply exit the hypervisor. The system ought
to naturally split the operation in several continuations each of which does
either unmapping or mapping if it couldn't be done in a single one. Replaying
the instruction after decoding is disabled ought to be benign.

I haven't tried any of what I just wrote, so take it with with several tons of
salt though.

Do you know if Linux intentionally skips disabling decode? Or is it a bug?

> +            }
> +        }
> +        else
> +            return;
>      }
>  
>  
> @@ -610,6 +647,10 @@ static void cf_check bar_write(
>      }
>  
>      pci_conf_write32(pdev->sbdf, reg, val);
> +
> +    if ( reenable )
> +        /* Write-while-mapped: map the new BAR in p2m. OK to defer. */
> +        modify_bars(pdev, cmd, false);
>  }
>  
>  static void cf_check guest_mem_bar_write(const struct pci_dev *pdev,
>
> base-commit: 8e60d47cf0112c145b6b0e454d102b04c857db8c

Cheers,
Alejandro