linux-kernel - Re: [PATCH v2 0/3] KVM: x86: KVM_MEM_PCI

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Message-ID: <20200825212526.GC8235@xz-x1>
Date:   Tue, 25 Aug 2020 17:25:26 -0400
From:   Peter Xu <peterx@...hat.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:     kvm@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Michael Tsirkin <mst@...hat.com>,
        Julia Suvorova <jsuvorov@...hat.com>,
        Andy Lutomirski <luto@...nel.org>,
        Andrew Jones <drjones@...hat.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 0/3] KVM: x86: KVM_MEM_PCI_HOLE memory

On Fri, Aug 07, 2020 at 04:12:29PM +0200, Vitaly Kuznetsov wrote:
> When testing Linux kernel boot with QEMU q35 VM and direct kernel boot
> I observed 8193 accesses to PCI hole memory. When such exit is handled
> in KVM without exiting to userspace, it takes roughly 0.000001 sec.
> Handling the same exit in userspace is six times slower (0.000006 sec) so
> the overal; difference is 0.04 sec. This may be significant for 'microvm'
> ideas.

Sorry to comment so late, but just curious... have you looked at what's those
8000+ accesses to PCI holes and what they're used for?  What I can think of are
some port IO reads (e.g. upon vendor ID field) during BIOS to scan the devices
attached.  Though those should be far less than 8000+, and those should also be
pio rather than mmio.

If this is only an overhead for virt (since baremetal mmios should be fast),
I'm also thinking whether we can make it even better to skip those pci hole
reads.  Because we know we're virt, so it also gives us possibility that we may
provide those information in a better way than reading PCI holes in the guest?

Thanks,

-- 
Peter Xu