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AJvYcCXYThmI6kli58ZxjlAAKALCQeZ1JTx8IZlWtrIog1KDtbZGnVgJYXRf51MXFk3ceFKpDJSp1GzOzvXNT6NFmsU77+0Q X-Gm-Message-State: AOJu0Ywyb+9+eAcmOgLnTkoOMBA9V+ddBSHubpLj2p5nZtk06bQ6FDE3 0xwubTa+fRLMp5Vz41BDXdYBjynsAgbxnPo/ShyPNeiyabRxwXK0frmyqP8gEp+8LQ/ZyTXvrOU MLA== X-Google-Smtp-Source: AGHT+IF5TxICpEgJNNaxPl2At7Hpo2uBQDuQJeY4CGfqjHedHjuOWEpukoYm6rVl8rxhvO9f6W4tfL2aE8M= X-Received: from zagreus.c.googlers.com ([fda3:e722:ac3:cc00:7f:e700:c0a8:5c37]) (user=seanjc job=sendgmr) by 2002:a17:90a:f2d5:b0:2c8:55e9:7874 with SMTP id 98e67ed59e1d1-2d392214b22mr1604a91.0.1723476132491; Mon, 12 Aug 2024 08:22:12 -0700 (PDT) Date: Mon, 12 Aug 2024 08:22:11 -0700 In-Reply-To: Precedence: bulk X-Mailing-List: kvm@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 References: <20240809194335.1726916-1-seanjc@google.com> <20240809194335.1726916-20-seanjc@google.com> Message-ID: Subject: Re: [PATCH 19/22] KVM: x86/mmu: Add infrastructure to allow walking rmaps outside of mmu_lock From: Sean Christopherson To: Lai Jiangshan Cc: Paolo Bonzini , kvm@vger.kernel.org, linux-kernel@vger.kernel.org, Oliver Upton , Marc Zyngier , Peter Xu , James Houghton Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: quoted-printable On Mon, Aug 12, 2024, Lai Jiangshan wrote: > On Sat, Aug 10, 2024 at 3:49=E2=80=AFAM Sean Christopherson wrote: >=20 > > + > > +static unsigned long kvm_rmap_lock(struct kvm_rmap_head *rmap_head) > > +{ > > + unsigned long old_val, new_val; > > + > > + old_val =3D READ_ONCE(rmap_head->val); > > + if (!old_val) > > + return 0; > > + > > + do { > > + /* > > + * If the rmap is locked, wait for it to be unlocked be= fore > > + * trying acquire the lock, e.g. to bounce the cache li= ne. > > + */ > > + while (old_val & KVM_RMAP_LOCKED) { > > + old_val =3D READ_ONCE(rmap_head->val); > > + cpu_relax(); >=20 > The sequence of these two lines of code can be improved. Oh yeah, duh, re-read after PAUSE, not before. Definitely holler if you have any alternative ideas for walking rmaps witho= ut taking mmu_lock, I guarantee you've spent more time than me thinking about = the shadow MMU :-)