From: Paolo Bonzini <pbonzini@redhat.com>
To: Marc Zyngier <maz@kernel.org>, Oliver Upton <oupton@google.com>
Cc: kvm@vger.kernel.org, Sean Christopherson <seanjc@google.com>,
Peter Shier <pshier@google.com>,
Raghavendra Rao Anata <rananta@google.com>,
David Matlack <dmatlack@google.com>,
kvmarm@lists.cs.columbia.edu, Jim Mattson <jmattson@google.com>
Subject: Re: [PATCH 02/10] KVM: arm64: Implement initial support for KVM_CAP_SYSTEM_COUNTER_STATE
Date: Thu, 10 Jun 2021 08:26:03 +0200 [thread overview]
Message-ID: <9c5f19f9-b303-573a-aa2a-cf5a91110db8@redhat.com> (raw)
In-Reply-To: <877dj3z68p.wl-maz@kernel.org>
On 09/06/21 12:23, Marc Zyngier wrote:
>> Implement initial support for KVM_{GET,SET}_SYSTEM_COUNTER_STATE ioctls
>> to migrate the value of CNTVOFF_EL2. These ioctls yield precise control
>> of the virtual counter-timers to userspace, allowing it to define its
>> own heuristics for managing vCPU offsets.
> I'm not really in favour of inventing a completely new API, for
> multiple reasons:
>
> - CNTVOFF is an EL2 concept. I'd rather not expose it as such as it
> becomes really confusing with NV (which does expose its own CNTVOFF
> via the ONE_REG interface)
>
> - You seem to allow each vcpu to get its own offset. I don't think
> that's right. The architecture defines that all PEs have the same
> view of the counters, and an EL1 guest should be given that
> illusion.
>
> - by having a parallel save/restore interface, you make it harder to
> reason about what happens with concurrent calls to both interfaces
>
> - the userspace API is already horribly bloated, and I'm not overly
> keen on adding more if we can avoid it.
>
> I'd rather you extend the current ONE_REG interface and make it modal,
> either allowing the restore of an absolute value or an offset for
> CNTVCT_EL0. This would also keep a consistent behaviour when restoring
> vcpus. The same logic would apply to the physical offset.
What about using KVM_GET/SET_DEVICE_ATTR? It makes sense that the guest
value for nested virt goes through ONE_REG, while the host value goes
through DEVICE_ATTR.
Paolo
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next prev parent reply other threads:[~2021-06-10 6:26 UTC|newest]
Thread overview: 23+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-06-08 21:47 [PATCH 00/10] KVM: Add idempotent controls for migrating system counter state Oliver Upton
2021-06-08 21:47 ` [PATCH 01/10] KVM: Introduce KVM_{GET, SET}_SYSTEM_COUNTER_STATE ioctls Oliver Upton
2021-06-08 21:47 ` [PATCH 02/10] KVM: arm64: Implement initial support for KVM_CAP_SYSTEM_COUNTER_STATE Oliver Upton
2021-06-08 21:55 ` Oliver Upton
2021-06-09 10:23 ` Marc Zyngier
2021-06-09 14:51 ` Oliver Upton
2021-06-10 6:54 ` Paolo Bonzini
2021-06-10 6:26 ` Paolo Bonzini [this message]
2021-06-08 21:47 ` [PATCH 03/10] selftests: KVM: Introduce system_counter_state_test Oliver Upton
2021-06-08 21:47 ` [PATCH 04/10] KVM: arm64: Add userspace control of the guest's physical counter Oliver Upton
2021-06-08 21:58 ` Oliver Upton
2021-06-08 21:47 ` [PATCH 05/10] selftests: KVM: Add test cases for physical counter offsetting Oliver Upton
2021-06-08 21:47 ` [PATCH 06/10] selftests: KVM: Add counter emulation benchmark Oliver Upton
2021-06-08 21:47 ` [PATCH 07/10] KVM: x86: Refactor tsc synchronization code Oliver Upton
2021-06-08 21:47 ` [PATCH 08/10] KVM: x86: Implement KVM_CAP_SYSTEM_COUNTER_STATE Oliver Upton
2021-06-08 21:47 ` [PATCH 09/10] selftests: KVM: Add support for x86 to system_counter_state_test Oliver Upton
2021-06-08 21:47 ` [PATCH 10/10] Documentation: KVM: Document KVM_{GET, SET}_SYSTEM_COUNTER_STATE ioctls Oliver Upton
2021-06-09 13:05 ` [PATCH 00/10] KVM: Add idempotent controls for migrating system counter state Paolo Bonzini
2021-06-09 15:11 ` Oliver Upton
2021-06-09 17:05 ` Paolo Bonzini
2021-06-09 22:04 ` Oliver Upton
2021-06-10 6:22 ` Paolo Bonzini
2021-06-10 6:53 ` Christian Borntraeger
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