linux-kernel - Re: [PATCH 00/44] KVM: Rework kvm

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Message-ID: <Y2qV+IGKyKQItMj2@google.com>
Date:   Tue, 8 Nov 2022 17:46:32 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Isaku Yamahata <isaku.yamahata@...il.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>, Marc Zyngier <maz@...nel.org>,
        Huacai Chen <chenhuacai@...nel.org>,
        Aleksandar Markovic <aleksandar.qemu.devel@...il.com>,
        Anup Patel <anup@...infault.org>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        Christian Borntraeger <borntraeger@...ux.ibm.com>,
        Janosch Frank <frankja@...ux.ibm.com>,
        Claudio Imbrenda <imbrenda@...ux.ibm.com>,
        Matthew Rosato <mjrosato@...ux.ibm.com>,
        Eric Farman <farman@...ux.ibm.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        James Morse <james.morse@....com>,
        Alexandru Elisei <alexandru.elisei@....com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Oliver Upton <oliver.upton@...ux.dev>,
        Atish Patra <atishp@...shpatra.org>,
        David Hildenbrand <david@...hat.com>, kvm@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.linux.dev,
        kvmarm@...ts.cs.columbia.edu, linux-mips@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org, kvm-riscv@...ts.infradead.org,
        linux-riscv@...ts.infradead.org, linux-s390@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Isaku Yamahata <isaku.yamahata@...el.com>,
        Fabiano Rosas <farosas@...ux.ibm.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Chao Gao <chao.gao@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Yuan Yao <yuan.yao@...el.com>
Subject: Re: [PATCH 00/44] KVM: Rework kvm_init() and hardware enabling

On Mon, Nov 07, 2022, Isaku Yamahata wrote:
> On Fri, Nov 04, 2022 at 08:27:14PM +0000,
> Sean Christopherson <seanjc@...gle.com> wrote:
> 
> > On Fri, Nov 04, 2022, Isaku Yamahata wrote:
> > > Thanks for the patch series. I the rebased TDX KVM patch series and it worked.
> > > Since cpu offline needs to be rejected in some cases(To keep at least one cpu
> > > on a package), arch hook for cpu offline is needed.
> > 
> > I hate to bring this up because I doubt there's a real use case for SUSPEND with
> > TDX, but the CPU offline path isn't just for true offlining of CPUs.  When the
> > system enters SUSPEND, only the initiating CPU goes through kvm_suspend()+kvm_resume(),
> > all responding CPUs go through CPU offline+online.  I.e. disallowing all CPUs from
> > going "offline" will prevent suspending the system.
> 
> The current TDX KVM implementation disallows CPU package from offline only when
> TDs are running.  If no TD is running, CPU offline is allowed.  So before
> SUSPEND, TDs need to be killed via systemd or something.  After killing TDs, the
> system can enter into SUSPEND state.

Ah, I assumed offlining was disallowed if TDX was enabled.

> > I don't see anything in the TDX series or the specs that suggests suspend+resume
> > is disallowed when TDX is enabled, so blocking that seems just as wrong as
> > preventing software from soft-offlining CPUs.
> 
> When it comes to SUSPEND, it means suspend-to-idle, ACPI S1, S3, or S4.
> suspend-to-idle doesn't require CPU offline.
> 
> Although CPU related spec doesn't mention about S3, the ACPI spec says
> 
>   7.4.2.2 System _S1 State (Sleeping with Processor Context Maintained)
>   The processor-complex context is maintained.
> 
>   7.4.2.4 System _S3 State or 7.4.2.5 System _S4 State
>   The processor-complex context is not maintained.
> 
> It's safe to say the processor context related to TDX is complex, I think.
> Let me summarize the situation. What do you think?
> 
> - While no TD running:
>   No additional limitation on CPU offline.
> 
> - On TD creation:
>   If any of whole cpu package is software offlined, TD creation fails.
>   Alternative: forcibly online necessary CPUs, create TD, and offline CPUs

The alternative isn't really viable because there's no way the kernel can guarantee
a CPU can be onlined, i.e. the kernel would need to fallback of disallowing TD
creation anyways.

> - TD running:
>   Although it's not required to keep all CPU packages online, keep CPU package
>   from offlining for TD destruction.
> 
> - TD destruction:
>   If any of whole cpu package is software offlined, TD destruction fails.
>   The current implementation prevents any cpu package from offlinining during
>   TD running.
>   Alternative:
>   - forcibly online necessary CPUs, destruct TD, and offline CPUs again and
>     allow CPU package to offline
>   - Stash TDX resources somewhere. When cpu packages are onlined, free those
>     release.
> 
> - On SUSPEND:
>   TODO: Allow CPU offline if S1 is requested.

Is this actually a TODO?  I assume the kernel doesn't actually try to offline
CPUs in this case, i.e. it Just Works.

>   - suspend-to-idle: nothing to do because cpu offline isn't required
>   - ACPI S1: Need to allow offline CPUs.  This can be implemented by referencing
>     suspend_state_t pm_suspend_target_state is PM_SUSPEND_TO_STANBY.
>   - ACPI S3/S4: refuse cpu offline.  The system needs to kill all TDs before
>     starting SUSPEND process. This is what is implemented.

Looks good, disallowing SUSPEND with active TDs is a reasonable tradeoff.  As
above, I highly doubt anyone actually cares.