linux-kernel - Re: [PATCH 5/6] KVM: x86/mmu: Protect kvm->memslots with a mutex

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Message-ID: <CANgfPd_S=LjEs+s2UzcHZKfUHf+n498eSbfidpXNFXjJT8kxzw@mail.gmail.com>
Date:   Wed, 28 Apr 2021 14:46:35 -0700
From:   Ben Gardon <bgardon@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     LKML <linux-kernel@...r.kernel.org>, kvm <kvm@...r.kernel.org>,
        Peter Xu <peterx@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Peter Shier <pshier@...gle.com>,
        Junaid Shahid <junaids@...gle.com>,
        Jim Mattson <jmattson@...gle.com>,
        Yulei Zhang <yulei.kernel@...il.com>,
        Wanpeng Li <kernellwp@...il.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Xiao Guangrong <xiaoguangrong.eric@...il.com>
Subject: Re: [PATCH 5/6] KVM: x86/mmu: Protect kvm->memslots with a mutex

On Wed, Apr 28, 2021 at 2:41 PM Paolo Bonzini <pbonzini@...hat.com> wrote:
>
> On 28/04/21 22:40, Ben Gardon wrote:
> > ... However with the locking you propose below, we might still run
> > into issues on a move or delete, which would mean we'd still need the
> > separate memory allocation for the rmaps array. Or we do some
> > shenanigans where we try to copy the rmap pointers from the other set
> > of memslots.
>
> If that's (almost) as easy as passing old to
> kvm_arch_prepare_memory_region, that would be totally okay.

Unfortunately it's not quite that easy because it's all the slots
_besides_ the one being modified where we'd need to copy the rmaps.

>
> > My only worry is the latency this could add to a nested VM launch, but
> > it seems pretty unlikely that that would be frequently coinciding with
> > a memslot change in practice.
>
> Right, memslot changes in practice occur only at boot and on hotplug.
> If that was a problem we could always make the allocation state
> off/in-progress/on, allowing to check the allocation state out of the
> lock.  This would only potentially slow down the first nested VM launch.
>
> Paolo
>