linux-kernel - Re: [PATCH v5 0/8] KVM: Various fixes and improvements around kicking vCPUs

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Message-ID: <a7e0bd04-bce9-9acb-1bb4-731d69181536@redhat.com>
Date:   Fri, 24 Sep 2021 13:05:38 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>, kvm@...r.kernel.org
Cc:     Sean Christopherson <seanjc@...gle.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        "Dr. David Alan Gilbert" <dgilbert@...hat.com>,
        Nitesh Narayan Lal <nitesh@...hat.com>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Maxim Levitsky <mlevitsk@...hat.com>,
        Eduardo Habkost <ehabkost@...hat.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 0/8] KVM: Various fixes and improvements around kicking
 vCPUs

On 03/09/21 09:51, Vitaly Kuznetsov wrote:
> Changes since v4 (Sean):
> - Add Reviewed-by: tag to PATCHes 3 and 5.
> - Drop unneeded 'vcpu' initialization from PATCH4.
> - Return -ENOMEM from kvm_init() when cpumask allocation fails and drop
>   unnecessary braces (PATCH 7).
> - Drop cpumask_available() check from kvm_kick_many_cpus() and convert
>   kvm_make_vcpu_request()'s parameter from 'cpumask_var_t' to
>   'struct cpumask *' (PATCH 8)
> 
> This series is a continuation to Sean's "[PATCH 0/2] VM: Fix a benign race
> in kicking vCPUs" work and v2 for my "KVM: Optimize
> kvm_make_vcpus_request_mask() a bit"/"KVM: x86: Fix stack-out-of-bounds
> memory access from ioapic_write_indirect()" patchset.

Now queued 3-4-5-7-8 as well, thanks.

Paolo

>  From Sean:
> 
> "Fix benign races when kicking vCPUs where the task doing the kicking can
> consume a stale vcpu->cpu.  The races are benign because of the
> impliciations of task migration with respect to interrupts and being in
> guest mode, but IMO they're worth fixing if only as an excuse to
> document the flows.
> 
> Patch 2 is a tangentially related cleanup to prevent future me from
> trying to get rid of the NULL check on the cpumask parameters, which
> _looks_ like it can't ever be NULL, but has a subtle edge case due to the
> way CONFIG_CPUMASK_OFFSTACK=y handles cpumasks."
> 
> Patch3 is a preparation to untangling kvm_make_all_cpus_request_except()
> and kvm_make_vcpus_request_mask().
> 
> Patch4 is a minor optimization for kvm_make_vcpus_request_mask() for big
> guests.
> 
> Patch5 is a minor cleanup.
> 
> Patch6 fixes a real problem with ioapic_write_indirect() KVM does
> out-of-bounds access to stack memory.
> 
> Patches7 and 8 get rid of dynamic cpumask allocation for kicking vCPUs.
> 
> Sean Christopherson (2):
>    KVM: Clean up benign vcpu->cpu data races when kicking vCPUs
>    KVM: KVM: Use cpumask_available() to check for NULL cpumask when
>      kicking vCPUs
> 
> Vitaly Kuznetsov (6):
>    KVM: x86: hyper-v: Avoid calling kvm_make_vcpus_request_mask() with
>      vcpu_mask==NULL
>    KVM: Optimize kvm_make_vcpus_request_mask() a bit
>    KVM: Drop 'except' parameter from kvm_make_vcpus_request_mask()
>    KVM: x86: Fix stack-out-of-bounds memory access from
>      ioapic_write_indirect()
>    KVM: Pre-allocate cpumasks for kvm_make_all_cpus_request_except()
>    KVM: Make kvm_make_vcpus_request_mask() use pre-allocated
>      cpu_kick_mask
> 
>   arch/x86/include/asm/kvm_host.h |   1 -
>   arch/x86/kvm/hyperv.c           |  18 ++---
>   arch/x86/kvm/ioapic.c           |  10 +--
>   arch/x86/kvm/x86.c              |   8 +--
>   include/linux/kvm_host.h        |   3 +-
>   virt/kvm/kvm_main.c             | 115 +++++++++++++++++++++++---------
>   6 files changed, 101 insertions(+), 54 deletions(-)
>