Re: [PATCH v8 05/13] vpci/header: implement guest BAR register handlers

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Thu, Jul 20, 2023 at 12:32:32AM +0000, Volodymyr Babchuk wrote:
> From: Oleksandr Andrushchenko <oleksandr_andrushchenko@xxxxxxxx>
> 
> Add relevant vpci register handlers when assigning PCI device to a domain
> and remove those when de-assigning. This allows having different
> handlers for different domains, e.g. hwdom and other guests.
> 
> Emulate guest BAR register values: this allows creating a guest view
> of the registers and emulates size and properties probe as it is done
> during PCI device enumeration by the guest.
> 
> All empty, IO and ROM BARs for guests are emulated by returning 0 on
> reads and ignoring writes: this BARs are special with this respect as
> their lower bits have special meaning, so returning default ~0 on read
> may confuse guest OS.
> 
> Memory decoding is initially disabled when used by guests in order to
> prevent the BAR being placed on top of a RAM region.

I'm kind of lost on this last sentence, as I don't see the patch
explicitly disabling PCI_COMMAND_MEMORY form the command register.  Is
that more of an expectation on the initial device state?

Maybe there should be some checking in that case then?

> 
> Signed-off-by: Oleksandr Andrushchenko <oleksandr_andrushchenko@xxxxxxxx>
> ---
> 
> Since v6:
> - unify the writing of the PCI_COMMAND register on the
>   error path into a label
> - do not introduce bar_ignore_access helper and open code
> - s/guest_bar_ignore_read/empty_bar_read
> - update error message in guest_bar_write
> - only setup empty_bar_read for IO if !x86
> Since v5:
> - make sure that the guest set address has the same page offset
>   as the physical address on the host
> - remove guest_rom_{read|write} as those just implement the default
>   behaviour of the registers not being handled
> - adjusted comment for struct vpci.addr field
> - add guest handlers for BARs which are not handled and will otherwise
>   return ~0 on read and ignore writes. The BARs are special with this
>   respect as their lower bits have special meaning, so returning ~0
>   doesn't seem to be right
> Since v4:
> - updated commit message
> - s/guest_addr/guest_reg
> Since v3:
> - squashed two patches: dynamic add/remove handlers and guest BAR
>   handler implementation
> - fix guest BAR read of the high part of a 64bit BAR (Roger)
> - add error handling to vpci_assign_device
> - s/dom%pd/%pd
> - blank line before return
> Since v2:
> - remove unneeded ifdefs for CONFIG_HAS_VPCI_GUEST_SUPPORT as more code
>   has been eliminated from being built on x86
> Since v1:
>  - constify struct pci_dev where possible
>  - do not open code is_system_domain()
>  - simplify some code3. simplify
>  - use gdprintk + error code instead of gprintk
>  - gate vpci_bar_{add|remove}_handlers with CONFIG_HAS_VPCI_GUEST_SUPPORT,
>    so these do not get compiled for x86
>  - removed unneeded is_system_domain check
>  - re-work guest read/write to be much simpler and do more work on write
>    than read which is expected to be called more frequently
>  - removed one too obvious comment
> ---
>  xen/drivers/vpci/header.c | 156 +++++++++++++++++++++++++++++++-------
>  xen/include/xen/vpci.h    |   3 +
>  2 files changed, 130 insertions(+), 29 deletions(-)
> 
> diff --git a/xen/drivers/vpci/header.c b/xen/drivers/vpci/header.c
> index 2780fcae72..5dc9b5338b 100644
> --- a/xen/drivers/vpci/header.c
> +++ b/xen/drivers/vpci/header.c
> @@ -457,6 +457,71 @@ static void cf_check bar_write(
>      pci_conf_write32(pdev->sbdf, reg, val);
>  }
>  
> +static void cf_check guest_bar_write(const struct pci_dev *pdev,
> +                                     unsigned int reg, uint32_t val, void 
> *data)
> +{
> +    struct vpci_bar *bar = data;
> +    bool hi = false;
> +    uint64_t guest_reg = bar->guest_reg;
> +
> +    if ( bar->type == VPCI_BAR_MEM64_HI )
> +    {
> +        ASSERT(reg > PCI_BASE_ADDRESS_0);
> +        bar--;
> +        hi = true;
> +    }
> +    else
> +    {
> +        val &= PCI_BASE_ADDRESS_MEM_MASK;
> +        val |= bar->type == VPCI_BAR_MEM32 ? PCI_BASE_ADDRESS_MEM_TYPE_32
> +                                           : PCI_BASE_ADDRESS_MEM_TYPE_64;
> +        val |= bar->prefetchable ? PCI_BASE_ADDRESS_MEM_PREFETCH : 0;
> +    }
> +
> +    guest_reg &= ~(0xffffffffull << (hi ? 32 : 0));
> +    guest_reg |= (uint64_t)val << (hi ? 32 : 0);
> +
> +    guest_reg &= ~(bar->size - 1) | ~PCI_BASE_ADDRESS_MEM_MASK;
> +
> +    /*
> +     * Make sure that the guest set address has the same page offset
> +     * as the physical address on the host or otherwise things won't work as
> +     * expected.
> +     */
> +    if ( (guest_reg & (~PAGE_MASK & PCI_BASE_ADDRESS_MEM_MASK)) !=
> +         (bar->addr & ~PAGE_MASK) )
> +    {
> +        gprintk(XENLOG_WARNING,
> +                "%pp: ignored BAR %zu write attempting to change page 
> offset\n",
> +                &pdev->sbdf, bar - pdev->vpci->header.bars + hi);
> +        return;
> +    }
> +
> +    bar->guest_reg = guest_reg;
> +}
> +
> +static uint32_t cf_check guest_bar_read(const struct pci_dev *pdev,
> +                                        unsigned int reg, void *data)
> +{
> +    const struct vpci_bar *bar = data;
> +    bool hi = false;
> +
> +    if ( bar->type == VPCI_BAR_MEM64_HI )
> +    {
> +        ASSERT(reg > PCI_BASE_ADDRESS_0);
> +        bar--;
> +        hi = true;
> +    }
> +
> +    return bar->guest_reg >> (hi ? 32 : 0);
> +}
> +
> +static uint32_t cf_check empty_bar_read(const struct pci_dev *pdev,
> +                                        unsigned int reg, void *data)
> +{
> +    return 0;
> +}
> +
>  static void cf_check rom_write(
>      const struct pci_dev *pdev, unsigned int reg, uint32_t val, void *data)
>  {
> @@ -517,6 +582,7 @@ static int cf_check init_bars(struct pci_dev *pdev)
>      struct vpci_header *header = &pdev->vpci->header;
>      struct vpci_bar *bars = header->bars;
>      int rc;
> +    bool is_hwdom = is_hardware_domain(pdev->domain);
>  
>      ASSERT(rw_is_locked(&pdev->domain->pci_lock));
>  
> @@ -558,13 +624,12 @@ static int cf_check init_bars(struct pci_dev *pdev)
>          if ( i && bars[i - 1].type == VPCI_BAR_MEM64_LO )
>          {
>              bars[i].type = VPCI_BAR_MEM64_HI;
> -            rc = vpci_add_register(pdev->vpci, vpci_hw_read32, bar_write, 
> reg,
> -                                   4, &bars[i]);
> +            rc = vpci_add_register(pdev->vpci,
> +                                   is_hwdom ? vpci_hw_read32 : 
> guest_bar_read,
> +                                   is_hwdom ? bar_write : guest_bar_write,
> +                                   reg, 4, &bars[i]);
>              if ( rc )
> -            {
> -                pci_conf_write16(pdev->sbdf, PCI_COMMAND, cmd);
> -                return rc;
> -            }
> +                goto fail;
>  
>              continue;
>          }
> @@ -573,6 +638,17 @@ static int cf_check init_bars(struct pci_dev *pdev)
>          if ( (val & PCI_BASE_ADDRESS_SPACE) == PCI_BASE_ADDRESS_SPACE_IO )
>          {
>              bars[i].type = VPCI_BAR_IO;
> +
> +#ifndef CONFIG_X86
> +            if ( !is_hwdom )
> +            {
> +                rc = vpci_add_register(pdev->vpci, empty_bar_read, NULL,
> +                                       reg, 4, &bars[i]);

For an empty BAR there's no need to pass &bars[i] around? (same for
all callers that setup empty_bar_read() handlers.

> +                if ( rc )
> +                    goto fail;
> +            }
> +#endif

This might be better done as an IS_ENABLED() check in the introduced
if condition.  Need a bit of a description as to why IO space BARs are
handled as empty BARs for domUs.

> +
>              continue;
>          }
>          if ( (val & PCI_BASE_ADDRESS_MEM_TYPE_MASK) ==
> @@ -584,14 +660,20 @@ static int cf_check init_bars(struct pci_dev *pdev)
>          rc = pci_size_mem_bar(pdev->sbdf, reg, &addr, &size,
>                                (i == num_bars - 1) ? PCI_BAR_LAST : 0);
>          if ( rc < 0 )
> -        {
> -            pci_conf_write16(pdev->sbdf, PCI_COMMAND, cmd);
> -            return rc;
> -        }
> +            goto fail;
>  
>          if ( size == 0 )
>          {
>              bars[i].type = VPCI_BAR_EMPTY;
> +
> +            if ( !is_hwdom )
> +            {
> +                rc = vpci_add_register(pdev->vpci, empty_bar_read, NULL,
> +                                       reg, 4, &bars[i]);
> +                if ( rc )
> +                    goto fail;
> +            }
> +
>              continue;
>          }
>  
> @@ -599,34 +681,50 @@ static int cf_check init_bars(struct pci_dev *pdev)
>          bars[i].size = size;
>          bars[i].prefetchable = val & PCI_BASE_ADDRESS_MEM_PREFETCH;
>  
> -        rc = vpci_add_register(pdev->vpci, vpci_hw_read32, bar_write, reg, 4,
> -                               &bars[i]);
> +        rc = vpci_add_register(pdev->vpci,
> +                               is_hwdom ? vpci_hw_read32 : guest_bar_read,
> +                               is_hwdom ? bar_write : guest_bar_write,
> +                               reg, 4, &bars[i]);
>          if ( rc )
> -        {
> -            pci_conf_write16(pdev->sbdf, PCI_COMMAND, cmd);
> -            return rc;
> -        }
> +            goto fail;
>      }
>  
> -    /* Check expansion ROM. */
> -    rc = pci_size_mem_bar(pdev->sbdf, rom_reg, &addr, &size, PCI_BAR_ROM);
> -    if ( rc > 0 && size )
> +    /* Check expansion ROM: we do not handle ROM for guests. */

Is there any specific reason for not handling ROM BAR for guests?

> +    if ( is_hwdom )
>      {
> -        struct vpci_bar *rom = &header->bars[num_bars];
> +        rc = pci_size_mem_bar(pdev->sbdf, rom_reg, &addr, &size, 
> PCI_BAR_ROM);
> +        if ( rc > 0 && size )
> +        {
> +            struct vpci_bar *rom = &header->bars[num_bars];
>  
> -        rom->type = VPCI_BAR_ROM;
> -        rom->size = size;
> -        rom->addr = addr;
> -        header->rom_enabled = pci_conf_read32(pdev->sbdf, rom_reg) &
> -                              PCI_ROM_ADDRESS_ENABLE;
> +            rom->type = VPCI_BAR_ROM;
> +            rom->size = size;
> +            rom->addr = addr;
> +            header->rom_enabled = pci_conf_read32(pdev->sbdf, rom_reg) &
> +                                  PCI_ROM_ADDRESS_ENABLE;
>  
> -        rc = vpci_add_register(pdev->vpci, vpci_hw_read32, rom_write, 
> rom_reg,
> -                               4, rom);
> -        if ( rc )
> -            rom->type = VPCI_BAR_EMPTY;
> +            rc = vpci_add_register(pdev->vpci, vpci_hw_read32, rom_write,
> +                                   rom_reg, 4, rom);
> +            if ( rc )
> +                rom->type = VPCI_BAR_EMPTY;
> +        }
> +    }
> +    else
> +    {
> +        if ( !is_hwdom )

Extra !is_hwdown?  The condition on the outer if is already is_hwdom,
and this is the else branch.

> +        {
> +            rc = vpci_add_register(pdev->vpci, empty_bar_read, NULL,
> +                                   rom_reg, 4, &header->bars[num_bars]);
> +            if ( rc )
> +                goto fail;
> +        }
>      }
>  
>      return (cmd & PCI_COMMAND_MEMORY) ? modify_bars(pdev, cmd, false) : 0;
> +
> + fail:
> +    pci_conf_write16(pdev->sbdf, PCI_COMMAND, cmd);
> +    return rc;

It might have been better for the usage of the fail label to be
introduced in a pre-patch, as there would then be less changes here
(and the pre-patch would be a non-functional change).

Thanks, Roger.