From: Sean Christopherson <seanjc@google.com>
To: Vitaly Kuznetsov <vkuznets@redhat.com>
Cc: kvm@vger.kernel.org, Paolo Bonzini <pbonzini@redhat.com>,
Wanpeng Li <wanpengli@tencent.com>,
Jim Mattson <jmattson@google.com>
Subject: Re: [PATCH RFC 0/4] KVM: x86: Drastically raise KVM_USER_MEM_SLOTS limit
Date: Fri, 15 Jan 2021 08:45:43 -0800 [thread overview]
Message-ID: <YAHGt3zIUpNbJQm7@google.com> (raw)
In-Reply-To: <87zh1a5fuj.fsf@vitty.brq.redhat.com>
On Fri, Jan 15, 2021, Vitaly Kuznetsov wrote:
> Sean Christopherson <seanjc@google.com> writes:
>
> > On Fri, Jan 15, 2021, Vitaly Kuznetsov wrote:
> >> Longer version:
> >>
> >> Current KVM_USER_MEM_SLOTS limit (509) can be a limiting factor for some
> >> configurations. In particular, when QEMU tries to start a Windows guest
> >> with Hyper-V SynIC enabled and e.g. 256 vCPUs the limit is hit as SynIC
> >> requires two pages per vCPU and the guest is free to pick any GFN for
> >> each of them, this fragments memslots as QEMU wants to have a separate
> >> memslot for each of these pages (which are supposed to act as 'overlay'
> >> pages).
> >
> > What exactly does QEMU do on the backend? I poked around the code a bit, but
> > didn't see anything relevant.
> >
>
> In QEMU's terms it registers memory sub-regions for these two pages (see
> synic_update() in hw/hyperv/hyperv.c). Memory for these page-sized
> sub-regions is allocated separately so in KVM terms they become
> page-sized slots and previously continuous 'system memory' slot breaks
> into several slots.
Doh, I had a super stale version checked out (2.9.50), no wonder I couldn't find
anything.
Isn't the memslot approach inherently flawed in that the SynIC is per-vCPU, but
memslots are per-VM? E.g. if vCPU1 accesses vCPU0's SynIC GPA, I would expect
that to access real memory, not the overlay. Or is there more QEMU magic going
on that I'm missing?
next prev parent reply other threads:[~2021-01-15 16:46 UTC|newest]
Thread overview: 19+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-01-15 13:18 [PATCH RFC 0/4] KVM: x86: Drastically raise KVM_USER_MEM_SLOTS limit Vitaly Kuznetsov
2021-01-15 13:18 ` [PATCH RFC 1/4] KVM: x86: Drop redundant KVM_MEM_SLOTS_NUM definition Vitaly Kuznetsov
2021-01-15 16:47 ` Sean Christopherson
2021-01-15 13:18 ` [PATCH RFC 2/4] KVM: mips: Drop KVM_PRIVATE_MEM_SLOTS definition Vitaly Kuznetsov
2021-01-15 13:18 ` [PATCH RFC 3/4] KVM: Define KVM_USER_MEM_SLOTS in arch-neutral include/linux/kvm_host.h Vitaly Kuznetsov
2021-01-15 17:05 ` Sean Christopherson
2021-01-18 9:52 ` Vitaly Kuznetsov
2021-01-19 17:20 ` Sean Christopherson
2021-01-20 11:34 ` Igor Mammedov
2021-01-20 12:02 ` Vitaly Kuznetsov
2021-01-20 17:25 ` Sean Christopherson
2021-01-15 13:18 ` [PATCH RFC 4/4] KVM: x86: Stop limiting KVM_USER_MEM_SLOTS Vitaly Kuznetsov
2021-01-15 16:03 ` [PATCH RFC 0/4] KVM: x86: Drastically raise KVM_USER_MEM_SLOTS limit Sean Christopherson
2021-01-15 16:23 ` Vitaly Kuznetsov
2021-01-15 16:45 ` Sean Christopherson [this message]
2021-01-15 18:47 ` Maciej S. Szmigiero
2021-01-27 17:55 ` Vitaly Kuznetsov
2021-01-27 22:26 ` Maciej S. Szmigiero
2021-01-28 8:47 ` Vitaly Kuznetsov
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