[Qemu-devel] Re: [PATCH] qemu/pci: optimize pci config handling

["Michael S. Tsirkin" <mst@redhat.com>]

On Wed, Oct 07, 2009 at 03:01:39PM +0200, Paolo Bonzini wrote:
> On 10/07/2009 02:33 PM, Michael S. Tsirkin wrote:
>> There's no need to save all of config space before each config cycle:
>> just the 64 byte header is enough for our purposes.  This will become
>> more important as we add pci express support, which has 4K config space.
>
> You can even go a step further and save it only if something is actually  
> being changed.  Untested though.

I'm actively trying to get out of range-checking address.
What you porpose here is certainly more code than we had.
So why is this a good idea?

> Not-quite-signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
>
> Paolo

> diff --git a/hw/pci.c b/hw/pci.c
> index 41e99a9..f9959fc 100644
> --- a/hw/pci.c
> +++ b/hw/pci.c
> @@ -541,19 +541,26 @@ uint32_t pci_default_read_config(PCIDevice *d,
>  
>  void pci_default_write_config(PCIDevice *d, uint32_t addr, uint32_t val, int l)
>  {
> -    uint8_t orig[PCI_CONFIG_SPACE_SIZE];
> +    uint8_t orig[PCI_CONFIG_HEADER_SIZE];
>      int i;
>  
> -    /* not efficient, but simple */
> -    memcpy(orig, d->config, PCI_CONFIG_SPACE_SIZE);
> -    for(i = 0; i < l && addr < PCI_CONFIG_SPACE_SIZE; val >>= 8, ++i, ++addr) {
> -        uint8_t wmask = d->wmask[addr];
> -        d->config[addr] = (d->config[addr] & ~wmask) | (val & wmask);
> +    /* not efficient, but simple.  If modifying the header, save it so we
> +       can compare its contents later.  */
> +    if (addr < sizeof orig) {
> +        memcpy(orig, d->config, sizeof orig);
> +    }
> +
> +    for(i = 0; i < l && addr+i < PCI_CONFIG_SPACE_SIZE; val >>= 8, ++i) {
> +        uint8_t wmask = d->wmask[addr+i];
> +        d->config[addr+i] = (d->config[addr+i] & ~wmask) | (val & wmask);
> +    }
> +
> +    if (addr < sizeof orig) {
> +        if (memcmp(orig + PCI_BASE_ADDRESS_0, d->config + PCI_BASE_ADDRESS_0, 24)
> +            || ((orig[PCI_COMMAND] ^ d->config[PCI_COMMAND])
> +                & (PCI_COMMAND_MEMORY | PCI_COMMAND_IO)))
> +            pci_update_mappings(d);
>      }
> -    if (memcmp(orig + PCI_BASE_ADDRESS_0, d->config + PCI_BASE_ADDRESS_0, 24)
> -        || ((orig[PCI_COMMAND] ^ d->config[PCI_COMMAND])
> -            & (PCI_COMMAND_MEMORY | PCI_COMMAND_IO)))
> -        pci_update_mappings(d);
>  }
>  
>  void pci_data_write(void *opaque, uint32_t addr, uint32_t val, int len)