Re: [PATCH v5 06/14] vpci/header: implement guest BAR register handlers

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

A couple more comments I realized while walking the dog.

On Thu, Nov 25, 2021 at 01:02:43PM +0200, Oleksandr Andrushchenko 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.
> 
> ROM BAR is only handled for the hardware domain and for guest domains
> there is a stub: at the moment PCI expansion ROM handling is supported
> for x86 only and it might not be used by other architectures without
> emulating x86. Other use-cases may include using that expansion ROM before
> Xen boots, hence no emulation is needed in Xen itself. Or when a guest
> wants to use the ROM code which seems to be rare.
> 
> Signed-off-by: Oleksandr Andrushchenko <oleksandr_andrushchenko@xxxxxxxx>
> ---
> 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 | 72 +++++++++++++++++++++++++++++++++++----
>  xen/include/xen/vpci.h    |  3 ++
>  2 files changed, 69 insertions(+), 6 deletions(-)
> 
> diff --git a/xen/drivers/vpci/header.c b/xen/drivers/vpci/header.c
> index ba333fb2f9b0..8880d34ebf8e 100644
> --- a/xen/drivers/vpci/header.c
> +++ b/xen/drivers/vpci/header.c
> @@ -433,6 +433,48 @@ static void bar_write(const struct pci_dev *pdev, 
> unsigned int reg,
>      pci_conf_write32(pdev->sbdf, reg, val);
>  }
>  
> +static void guest_bar_write(const struct pci_dev *pdev, unsigned int reg,
> +                            uint32_t val, void *data)
> +{
> +    struct vpci_bar *bar = data;
> +    bool hi = false;
> +
> +    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;
> +    }
> +
> +    bar->guest_reg &= ~(0xffffffffull << (hi ? 32 : 0));
> +    bar->guest_reg |= (uint64_t)val << (hi ? 32 : 0);
> +
> +    bar->guest_reg &= ~(bar->size - 1) | ~PCI_BASE_ADDRESS_MEM_MASK;

You need to assert 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. Ie: guest_addr & ~PAGE_MASK == addr & ~PAGE_MASK.

> +}
> +
> +static uint32_t 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 void rom_write(const struct pci_dev *pdev, unsigned int reg,
>                        uint32_t val, void *data)
>  {
> @@ -481,6 +523,17 @@ static void rom_write(const struct pci_dev *pdev, 
> unsigned int reg,
>          rom->addr = val & PCI_ROM_ADDRESS_MASK;
>  }
>  
> +static void guest_rom_write(const struct pci_dev *pdev, unsigned int reg,
> +                            uint32_t val, void *data)
> +{
> +}
> +
> +static uint32_t guest_rom_read(const struct pci_dev *pdev, unsigned int reg,
> +                               void *data)
> +{
> +    return 0xffffffff;
> +}
> +
>  static int init_bars(struct pci_dev *pdev)
>  {
>      uint16_t cmd;
> @@ -489,6 +542,7 @@ static int 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);
>  
>      switch ( pci_conf_read8(pdev->sbdf, PCI_HEADER_TYPE) & 0x7f )
>      {
> @@ -528,8 +582,10 @@ static int 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);
> @@ -569,8 +625,10 @@ static int 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]);

You need to initialize guest_reg to the physical host value also.

>          if ( rc )
>          {
>              pci_conf_write16(pdev->sbdf, PCI_COMMAND, cmd);
> @@ -590,8 +648,10 @@ static int init_bars(struct pci_dev *pdev)
>          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);
> +        rc = vpci_add_register(pdev->vpci,
> +                               is_hwdom ? vpci_hw_read32 : guest_rom_read,
> +                               is_hwdom ? rom_write : guest_rom_write,
> +                               rom_reg, 4, rom);
>          if ( rc )
>              rom->type = VPCI_BAR_EMPTY;

Also memory decoding needs to be initially disabled when used by
guests, in order to prevent the BAR being placed on top of a RAM
region. The guest physmap will be different from the host one, so it's
possible for BARs to end up placed on top of RAM regions initially
until the firmware or OS places them at a suitable address.

Thanks, Roger.