linux-kernel - Re: [PATCH v2 20/28] KVM: x86/mmu: Use atomic ops to set SPTEs in TDP MMU map

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Message-ID: <YB3iUde728MPiuo9@google.com>
Date:   Fri, 5 Feb 2021 16:26:57 -0800
From:   Sean Christopherson <seanjc@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Ben Gardon <bgardon@...gle.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org, Peter Xu <peterx@...hat.com>,
        Peter Shier <pshier@...gle.com>,
        Peter Feiner <pfeiner@...gle.com>,
        Junaid Shahid <junaids@...gle.com>,
        Jim Mattson <jmattson@...gle.com>,
        Yulei Zhang <yulei.kernel@...il.com>,
        Wanpeng Li <kernellwp@...il.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Xiao Guangrong <xiaoguangrong.eric@...il.com>
Subject: Re: [PATCH v2 20/28] KVM: x86/mmu: Use atomic ops to set SPTEs in
 TDP MMU map

On Wed, Feb 03, 2021, Paolo Bonzini wrote:
> On 02/02/21 19:57, Ben Gardon wrote:
> > To prepare for handling page faults in parallel, change the TDP MMU
> > page fault handler to use atomic operations to set SPTEs so that changes
> > are not lost if multiple threads attempt to modify the same SPTE.
> > 
> > Reviewed-by: Peter Feiner <pfeiner@...gle.com>
> > Signed-off-by: Ben Gardon <bgardon@...gle.com>
> > 
> > ---
> > 
> > v1 -> v2
> > - Rename "atomic" arg to "shared" in multiple functions
> > - Merged the commit that protects the lists of TDP MMU pages with a new
> >    lock
> > - Merged the commits to add an atomic option for setting SPTEs and to
> >    use that option in the TDP MMU page fault handler
> 
> I'll look at the kernel test robot report if nobody beats me to it.

It's just a vanilla i386 compilation issue, the xchg() is on an 8-byte value.

We could fudge around it via #ifdef around the xchg().  Making all of tdp_mmu.c
x86-64 only would be nice to avoid future annoyance, though the number of stubs
required would be painful...