From: Vitaly Kuznetsov <vkuznets@redhat.com>
To: Sean Christopherson <sean.j.christopherson@intel.com>,
Peter Xu <peterx@redhat.com>
Cc: kvm@vger.kernel.org, linux-kernel@vger.kernel.org,
Michael Tsirkin <mst@redhat.com>,
Julia Suvorova <jsuvorov@redhat.com>,
Paolo Bonzini <pbonzini@redhat.com>,
Wanpeng Li <wanpengli@tencent.com>,
Jim Mattson <jmattson@google.com>,
x86@kernel.org
Subject: Re: [PATCH RFC 0/5] KVM: x86: KVM_MEM_ALLONES memory
Date: Fri, 15 May 2020 10:42:12 +0200 [thread overview]
Message-ID: <87d075wpwb.fsf@vitty.brq.redhat.com> (raw)
In-Reply-To: <20200514233208.GI15847@linux.intel.com>
Sean Christopherson <sean.j.christopherson@intel.com> writes:
> On Thu, May 14, 2020 at 07:22:50PM -0400, Peter Xu wrote:
>> On Thu, May 14, 2020 at 03:56:24PM -0700, Sean Christopherson wrote:
>> > On Thu, May 14, 2020 at 06:05:16PM -0400, Peter Xu wrote:
>> > > E.g., shm_open() with a handle and fill one 0xff page, then remap it to
>> > > anywhere needed in QEMU?
>> >
>> > Mapping that 4k page over and over is going to get expensive, e.g. each
>> > duplicate will need a VMA and a memslot, plus any PTE overhead. If the
>> > total sum of the holes is >2mb it'll even overflow the mumber of allowed
>> > memslots.
>>
>> What's the PTE overhead you mentioned? We need to fill PTEs one by one on
>> fault even if the page is allocated in the kernel, am I right?
>
> It won't require host PTEs for every page if it's a kernel page. I doubt
> PTEs are a significant overhead, especially compared to memslots, but it's
> still worth considering.
>
> My thought was to skimp on both host PTEs _and_ KVM SPTEs by always sending
> the PCI hole accesses down the slow MMIO path[*].
>
> [*] https://lkml.kernel.org/r/20200514194624.GB15847@linux.intel.com
>
If we drop 'aggressive' patch from this patchset we can probably get
away with KVM_MEM_READONLY and userspace VMAs but this will only help us
to save some memory, it won't speed things up.
>> 4K is only an example - we can also use more pages as the template. However I
>> guess the kvm memslot count could be a limit.. Could I ask what's the normal
>> size of this 0xff region, and its distribution?
Julia/Michael, could you please provide some 'normal' configuration for
a Q35 machine and its PCIe config space?
--
Vitaly
next prev parent reply other threads:[~2020-05-15 8:42 UTC|newest]
Thread overview: 18+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-05-14 18:05 [PATCH RFC 0/5] KVM: x86: KVM_MEM_ALLONES memory Vitaly Kuznetsov
2020-05-14 18:05 ` [PATCH RFC 1/5] KVM: rename labels in kvm_init() Vitaly Kuznetsov
2020-05-14 18:05 ` [PATCH RFC 2/5] KVM: x86: introduce KVM_MEM_ALLONES memory Vitaly Kuznetsov
2020-05-14 19:18 ` Sean Christopherson
2020-05-15 8:24 ` Vitaly Kuznetsov
2020-05-14 18:05 ` [PATCH RFC 3/5] KVM: x86: move kvm_vcpu_gfn_to_memslot() out of try_async_pf() Vitaly Kuznetsov
2020-05-14 18:05 ` [PATCH RFC 4/5] KVM: x86: aggressively map PTEs in KVM_MEM_ALLONES slots Vitaly Kuznetsov
2020-05-14 19:46 ` Sean Christopherson
2020-05-15 8:36 ` Vitaly Kuznetsov
2020-05-15 13:58 ` Sean Christopherson
2020-05-14 18:05 ` [PATCH RFC 5/5] KVM: selftests: add KVM_MEM_ALLONES test Vitaly Kuznetsov
2020-05-14 22:05 ` [PATCH RFC 0/5] KVM: x86: KVM_MEM_ALLONES memory Peter Xu
2020-05-14 22:56 ` Sean Christopherson
2020-05-14 23:22 ` Peter Xu
2020-05-14 23:32 ` Sean Christopherson
2020-05-15 8:42 ` Vitaly Kuznetsov [this message]
2020-05-15 1:03 ` Andy Lutomirski
2020-05-15 11:15 ` Peter Xu
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