Re: [PATCH] KVM: x86: Use MMCONFIG for all PCI config space accesses

Andy Shevchenko
2020-07-30T19:50:50Z

On Thu, Jul 30, 2020 at 10:37 PM Julia Suvorova <jusual@redhat.com> wrote:
>
> Using MMCONFIG instead of I/O ports cuts the number of config space
> accesses in half, which is faster on KVM and opens the door for
> additional optimizations such as Vitaly's "[PATCH 0/3] KVM: x86: KVM
> MEM_PCI_HOLE memory":

> https://lore.kernel.org/kvm/20200728143741.2718593-1-vkuznets@redhat.com

You may use Link: tag for this.

> However, this change will not bring significant performance improvement
> unless it is running on x86 within a hypervisor. Moreover, allowing
> MMCONFIG access for addresses < 256 can be dangerous for some devices:
> see commit a0ca99096094 ("PCI x86: always use conf1 to access config
> space below 256 bytes"). That is why a special feature flag is needed.
>
> Introduce KVM_FEATURE_PCI_GO_MMCONFIG, which can be enabled when the
> configuration is known to be safe (e.g. in QEMU).

...

> +static int __init kvm_pci_arch_init(void)
> +{
> + if (raw_pci_ext_ops &&
> + kvm_para_has_feature(KVM_FEATURE_PCI_GO_MMCONFIG)) {

Better to use traditional pattern, i.e.
if (not_supported)
return bail_out;

...do useful things...
return 0;

> + pr_info("PCI: Using MMCONFIG for base access\n");
> + raw_pci_ops = raw_pci_ext_ops;
> + return 0;
> + }

> + return 1;

Hmm... I don't remember what positive codes means there. Perhaps you
need to return a rather error code?

> +}

--
With Best Regards,
Andy Shevchenko