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Date:   Tue, 26 Sep 2023 19:28:45 +0200
From:   David Woodhouse <dwmw2@infradead.org>
To:     Paolo Bonzini <pbonzini@redhat.com>,
        Alexander Graf <graf@amazon.de>
CC:     kvm@vger.kernel.org, Peter Zijlstra <peterz@infradead.org>,
        Sean Christopherson <seanjc@google.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        Dave Hansen <dave.hansen@linux.intel.com>, x86@kernel.org,
        "H. Peter Anvin" <hpa@zytor.com>, linux-kernel@vger.kernel.org,
        Nicolas Saenz Julienne <nsaenz@amazon.es>,
        "Griffoul, Fred" <fgriffo@amazon.com>
Subject: =?US-ASCII?Q?Re=3A_=5BRFC=5D_KVM=3A_x86=3A_Allow_userspace_exi?= =?US-ASCII?Q?t_on_HLT_and_MWAIT=2C_else_yield_on_MWAIT?=
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On 26 September 2023 19:20:24 CEST, Paolo Bonzini <pbonzini@redhat=2Ecom> =
wrote:
>On Sat, Sep 23, 2023 at 6:44=E2=80=AFPM Alexander Graf <graf@amazon=2Ede>=
 wrote:
>> On 23=2E09=2E23 11:24, Paolo Bonzini wrote:
>> > Why do you need it?  You can just use KVM_RUN to go to sleep, and if =
you
>> > get another job you kick out the vCPU with pthread_kill=2E  (I also d=
idn't
>> > get the VSM reference)=2E
>>
>> With the original VSM patches, we used to make a vCPU aware of the fact
>> that it can morph into one of many VTLs=2E That approach turned out to =
be
>> insanely intrusive and fragile and so we're currently reimplementing
>> everything as VTLs as vCPUs=2E That allows us to move the majority of V=
SM
>> functionality to user space=2E Everything we've seen so far looks as if
>> there is no real performance loss with that approach=2E
>
>Yes, that was also what I remember, sharing the FPU somehow while
>having separate vCPU file descriptors=2E
>
>> One small problem with that is that now user space is responsible for
>> switching between VTLs: It determines which VTL is currently running an=
d
>> leaves all others (read: all other vCPUs) as stopped=2E That means if y=
ou
>> are running happily in KVM_RUN in VTL0 and VTL1 gets an interrupt, user
>> space needs to stop VTL0 and unpause VTL1 until it triggers VTL_RETURN
>> at which point VTL1 stops execution and VTL0 runs again=2E
>
>That's with IPIs in VTL1, right? I understand now=2E My idea was, since
>we need a link from VTL1 to VTL0 for the FPU, to use the same link to
>trigger a vmexit to userspace if source VTL > destination VTL=2E I am
>not sure how you would handle the case where the destination vCPU is
>not running; probably by detecting the IPI when VTL0 restarts on the
>destination vCPU?
>
>In any case, making vCPUs poll()-able is sensible=2E

Thinking about this a bit more, even for HLT it probably isn't just as sim=
ple as checking for mp_state changes=2E If there's a REQ_EVENT outstanding =
for something like a timer delivery, that won't get handled and the IRQ act=
ually delivered to the local APIC until the vCPU is actually *run*, will it=
?