From: Peter Xu <peterx@redhat.com>
To: Vitaly Kuznetsov <vkuznets@redhat.com>
Cc: kvm@vger.kernel.org, Paolo Bonzini <pbonzini@redhat.com>,
Sean Christopherson <sean.j.christopherson@intel.com>,
Wanpeng Li <wanpengli@tencent.com>,
Jim Mattson <jmattson@google.com>,
Michael Tsirkin <mst@redhat.com>,
Julia Suvorova <jsuvorov@redhat.com>,
Andy Lutomirski <luto@kernel.org>,
Andrew Jones <drjones@redhat.com>,
linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2 0/3] KVM: x86: KVM_MEM_PCI_HOLE memory
Date: Tue, 25 Aug 2020 17:25:26 -0400 [thread overview]
Message-ID: <20200825212526.GC8235@xz-x1> (raw)
In-Reply-To: <20200807141232.402895-1-vkuznets@redhat.com>
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
next prev parent reply other threads:[~2020-08-25 21:25 UTC|newest]
Thread overview: 31+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-08-07 14:12 [PATCH v2 0/3] KVM: x86: KVM_MEM_PCI_HOLE memory Vitaly Kuznetsov
2020-08-07 14:12 ` [PATCH v2 1/3] KVM: x86: move kvm_vcpu_gfn_to_memslot() out of try_async_pf() Vitaly Kuznetsov
2020-08-14 1:40 ` Sean Christopherson
2020-09-01 14:15 ` Vitaly Kuznetsov
2020-09-04 3:47 ` Sean Christopherson
2020-08-07 14:12 ` [PATCH v2 2/3] KVM: x86: introduce KVM_MEM_PCI_HOLE memory Vitaly Kuznetsov
2020-08-14 2:31 ` Sean Christopherson
2020-08-14 14:30 ` Michael S. Tsirkin
2020-08-17 16:32 ` Sean Christopherson
2020-08-21 1:46 ` Michael S. Tsirkin
2020-08-22 3:19 ` Sean Christopherson
2020-09-01 14:39 ` Vitaly Kuznetsov
2020-09-03 9:35 ` Michael S. Tsirkin
2020-08-07 14:12 ` [PATCH v2 3/3] KVM: selftests: add KVM_MEM_PCI_HOLE test Vitaly Kuznetsov
2020-08-25 21:25 ` Peter Xu [this message]
2020-09-01 14:43 ` [PATCH v2 0/3] KVM: x86: KVM_MEM_PCI_HOLE memory Vitaly Kuznetsov
2020-09-01 20:00 ` Peter Xu
2020-09-02 8:59 ` Vitaly Kuznetsov
2020-09-04 6:12 ` Sean Christopherson
2020-09-04 7:29 ` Gerd Hoffmann
2020-09-04 16:00 ` Sean Christopherson
2020-09-07 8:37 ` Vitaly Kuznetsov
2020-09-07 11:32 ` Michael S. Tsirkin
2020-09-11 17:00 ` Sean Christopherson
2020-09-18 12:34 ` Michael S. Tsirkin
2020-09-21 17:21 ` Sean Christopherson
2020-09-07 10:52 ` Michael S. Tsirkin
2020-09-18 9:33 ` Gerd Hoffmann
2020-09-07 10:54 ` Michael S. Tsirkin
2020-09-18 8:49 ` Gerd Hoffmann
2020-09-04 7:20 ` Gerd Hoffmann
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