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Date:   Mon, 16 May 2022 14:08:08 +0000
From:   Sean Christopherson <seanjc@google.com>
To:     Maxim Levitsky <mlevitsk@redhat.com>
Cc:     Uros Bizjak <ubizjak@gmail.com>,
        Peter Zijlstra <peterz@infradead.org>, X86 ML <x86@kernel.org>,
        LKML <linux-kernel@vger.kernel.org>, kvm@vger.kernel.org,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        "H. Peter Anvin" <hpa@zytor.com>, Will Deacon <will@kernel.org>,
        Boqun Feng <boqun.feng@gmail.com>,
        Mark Rutland <mark.rutland@arm.com>,
        "Paul E. McKenney" <paulmck@kernel.org>,
        Marco Elver <elver@google.com>
Subject: Re: [PATCH] locking/atomic/x86: Introduce try_cmpxchg64
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On Mon, May 16, 2022, Maxim Levitsky wrote:
> On Wed, 2022-05-11 at 21:54 +0200, Uros Bizjak wrote:
> > On Wed, May 11, 2022 at 6:04 PM Sean Christopherson <seanjc@google.com> wrote:
> > > On Wed, May 11, 2022, Uros Bizjak wrote:
> > > > On Wed, May 11, 2022 at 9:54 AM Peter Zijlstra <peterz@infradead.org> wrote:
> > > > > Still, does 32bit actually support that stuff?
> > > > 
> > > > Unfortunately, it does:
> > > > 
> > > > kvm-intel-y        += vmx/vmx.o vmx/vmenter.o vmx/pmu_intel.o vmx/vmcs12.o \
> > > >                vmx/evmcs.o vmx/nested.o vmx/posted_intr.o
> > > > 
> > > > And when existing cmpxchg64 is substituted with cmpxchg, the
> > > > compilation dies for 32bits with:
> > > 
> > > ...
> > > 
> > > > > Anyway, your patch looks about right, but I find it *really* hard to
> > > > > care about 32bit code these days.
> > > > 
> > > > Thanks, this is also my sentiment, but I hope the patch will enable
> > > > better code and perhaps ease similar situation I have had elsewhere.
> > > 
> > > IMO, if we merge this it should be solely on the benefits to 64-bit code.  Yes,
> > > KVM still supports 32-bit kernels, but I'm fairly certain the only people that
> > > run 32-bit KVM are KVM developers.  32-bit KVM has been completely broken for
> > > multiple releases at least once, maybe twice, and no one ever complained.
> > 
> > Yes, the idea was to improve cmpxchg64 with the implementation of
> > try_cmpxchg64 for 64bit targets. However, the issue with 32bit targets
> > stood in the way, so the effort with 32-bit implementation was mainly
> > to unblock progression for 64-bit targets.
> 
> Would that allow tdp mmu to work on 32 bit?

From a purely technical perspective, there's nothing that prevents enabling the
TDP MMU on 32-bit kernels.  The TDP MMU is 64-bit only to simplify the implementation
and to reduce the maintenance and validation costs.