linux-kernel - Re: [PATCH] x86/kvm: do not setup pv tlb flush when not paravirtualized

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Date:   Mon, 3 Feb 2020 20:25:53 +0800
From:   Wanpeng Li <kernellwp@...il.com>
To:     Thadeu Lima de Souza Cascardo <cascardo@...onical.com>
Cc:     Vitaly Kuznetsov <vkuznets@...hat.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>, kvm <kvm@...r.kernel.org>
Subject: Re: [PATCH] x86/kvm: do not setup pv tlb flush when not paravirtualized

On Mon, 3 Feb 2020 at 18:31, Thadeu Lima de Souza Cascardo
<cascardo@...onical.com> wrote:
>
> On Mon, Feb 03, 2020 at 10:59:10AM +0100, Vitaly Kuznetsov wrote:
> > Thadeu Lima de Souza Cascardo <cascardo@...onical.com> writes:
> >
> > > kvm_setup_pv_tlb_flush will waste memory and print a misguiding message
> > > when KVM paravirtualization is not available.
> > >
> > > Intel SDM says that the when cpuid is used with EAX higher than the
> > > maximum supported value for basic of extended function, the data for the
> > > highest supported basic function will be returned.
> > >
> > > So, in some systems, kvm_arch_para_features will return bogus data,
> > > causing kvm_setup_pv_tlb_flush to detect support for pv tlb flush.
> > >
> > > Testing for kvm_para_available will work as it checks for the hypervisor
> > > signature.
> > >
> > > Besides, when the "nopv" command line parameter is used, it should not
> > > continue as well, as kvm_guest_init will no be called in that case.
> > >
> > > Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@...onical.com>
> > > ---
> > >  arch/x86/kernel/kvm.c | 3 +++
> > >  1 file changed, 3 insertions(+)
> > >
> > > diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
> > > index 81045aabb6f4..d817f255aed8 100644
> > > --- a/arch/x86/kernel/kvm.c
> > > +++ b/arch/x86/kernel/kvm.c
> > > @@ -736,6 +736,9 @@ static __init int kvm_setup_pv_tlb_flush(void)
> > >  {
> > >     int cpu;
> > >
> > > +   if (!kvm_para_available() || nopv)
> > > +           return 0;
> > > +
> > >     if (kvm_para_has_feature(KVM_FEATURE_PV_TLB_FLUSH) &&
> > >         !kvm_para_has_hint(KVM_HINTS_REALTIME) &&
> > >         kvm_para_has_feature(KVM_FEATURE_STEAL_TIME)) {
> >
> > The patch will fix the immediate issue, but why kvm_setup_pv_tlb_flush()
> > is just an arch_initcall() which will be executed regardless of the fact
> > if we are running on KVM or not?
> >
> > In Hyper-V we setup PV TLB flush from ms_hyperv_init_platform() -- which
> > only happens if Hyper-V platform was detected. Why don't we do it from
> > kvm_init_platform() in KVM?
> >
> > --
> > Vitaly
> >
>
> Because we can't call the allocator that early.
>
> Also, see the thread where this was "decided", the v6 of the original patch:
>
> https://lore.kernel.org/kvm/20171129162118.GA10661@flask/

A little change to this function.
https://lore.kernel.org/kvm/CANRm+CwK0Cg45mktda9Yz9fsjPCvtuB8O+fma5L3tV725ki1qw@mail.gmail.com
Testing is a great appreciated. (Still in vacation)

    Wanpeng