KVM Archive on lore.kernel.org
 help / color / Atom feed
From: Andy Shevchenko <andy.shevchenko@gmail.com>
To: Julia Suvorova <jusual@redhat.com>
Cc: "open list:VFIO DRIVER" <kvm@vger.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Bjorn Helgaas <helgaas@kernel.org>,
	"Michael S. Tsirkin" <mst@redhat.com>,
	Matthew Wilcox <willy@infradead.org>,
	Vitaly Kuznetsov <vkuznets@redhat.com>,
	Sean Christopherson <sean.j.christopherson@intel.com>,
	Paolo Bonzini <pbonzini@redhat.com>,
	Thomas Gleixner <tglx@linutronix.de>
Subject: Re: [PATCH] KVM: x86: Use MMCONFIG for all PCI config space accesses
Date: Thu, 30 Jul 2020 22:50:29 +0300
Message-ID: <CAHp75VcyRjAr3ugmAWYcKMrAeea6ioQOPfJnj-Srntdg_W8ScQ@mail.gmail.com> (raw)
In-Reply-To: <20200730193510.578309-1-jusual@redhat.com>

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

  reply index

Thread overview: 5+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-07-30 19:35 Julia Suvorova
2020-07-30 19:50 ` Andy Shevchenko [this message]
2020-07-31  9:22   ` Vitaly Kuznetsov
2020-07-31  9:41     ` Andy Shevchenko
2020-07-31 18:23     ` Julia Suvorova

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=CAHp75VcyRjAr3ugmAWYcKMrAeea6ioQOPfJnj-Srntdg_W8ScQ@mail.gmail.com \
    --to=andy.shevchenko@gmail.com \
    --cc=helgaas@kernel.org \
    --cc=jusual@redhat.com \
    --cc=kvm@vger.kernel.org \
    --cc=linux-kernel@vger.kernel.org \
    --cc=mst@redhat.com \
    --cc=pbonzini@redhat.com \
    --cc=sean.j.christopherson@intel.com \
    --cc=tglx@linutronix.de \
    --cc=vkuznets@redhat.com \
    --cc=willy@infradead.org \


* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

KVM Archive on lore.kernel.org

Archives are clonable:
	git clone --mirror https://lore.kernel.org/kvm/0 kvm/git/0.git

	# If you have public-inbox 1.1+ installed, you may
	# initialize and index your mirror using the following commands:
	public-inbox-init -V2 kvm kvm/ https://lore.kernel.org/kvm \
	public-inbox-index kvm

Example config snippet for mirrors

Newsgroup available over NNTP:

AGPL code for this site: git clone https://public-inbox.org/public-inbox.git