From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mail-pj1-f73.google.com (mail-pj1-f73.google.com [209.85.216.73]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 2937A254AF0 for ; Mon, 10 Feb 2025 22:41:41 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=209.85.216.73 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1739227303; cv=none; b=SOIv+PC/oozc6OQFWNV3M0qsAJfpG0oUSJ/4NgegIZL5IJHATkKpcC+s9ytc3UJVRweUMhNQpFW8NEoOebBsa/LN46reoJdyOmWs8X+Gj6ujxozuvyH+G2+IB+l36g36sYU8ckPUIbH8nYSo912TkKucd2gU6kuM7R88EhoggV8= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1739227303; c=relaxed/simple; bh=J2M5LBZIRA6dmTpu1re8X+PdJJRdN2XLIrkz2r8effU=; h=Date:In-Reply-To:Mime-Version:References:Message-ID:Subject:From: To:Cc:Content-Type; b=sq5r3247qKTuyq5xB358pzUNrNnqTgyzNWcrzwcdIZSHJ4Y5nqVvZJSdvfKge6R5foDzvAiBmjksHUzqKjdNWbSOetONxKYg2OL+BgGcLO7i1RAbKf6mnncPgaJhjaJQrK/aJufnJRewwPaPeaKAy8PZYRC0UXaVf5iSVh76PYA= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=reject dis=none) header.from=google.com; spf=pass smtp.mailfrom=flex--seanjc.bounces.google.com; dkim=pass (2048-bit key) header.d=google.com header.i=@google.com header.b=4rHaX52s; arc=none smtp.client-ip=209.85.216.73 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=reject dis=none) header.from=google.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=flex--seanjc.bounces.google.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=google.com header.i=@google.com header.b="4rHaX52s" Received: by mail-pj1-f73.google.com with SMTP id 98e67ed59e1d1-2fa228b4151so7597268a91.1 for ; Mon, 10 Feb 2025 14:41:41 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20230601; t=1739227301; x=1739832101; darn=vger.kernel.org; h=cc:to:from:subject:message-id:references:mime-version:in-reply-to :date:from:to:cc:subject:date:message-id:reply-to; bh=DZD6V+4k9UUTVu1q6+YHZha34NbYh40y70H3kTsa6mc=; b=4rHaX52sRJ4jIAyvJLAOq2hrrp9wiZ8j1Ner8REYgdlK5KyBt84oPCXh4eVtORkuRA B/F6jDotZn0UprV0Dx72Rboz34bU0vby3ajujLRXYG3bKph6HtPmXXT5o5h+cw/NuO55 39XnICl6g0LkImXReEMpZ7+LCr+aRA00nprKnMaIml6FWZZMP1tjlD10/10lkhIcOJxL gQyOM4wtNta1W8nd/vPM/Wuew7M7BeKkK4/cE4S43ZN8Fg88ehBMCrZNNOfhM7idxD8k Zop4d9snir/d0ugURk7zVutou/GDnh6eXTyyp8/UKCLBUxz1Bn7Kq63LHrW2IicZSUq3 ydfw== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1739227301; x=1739832101; h=cc:to:from:subject:message-id:references:mime-version:in-reply-to :date:x-gm-message-state:from:to:cc:subject:date:message-id:reply-to; bh=DZD6V+4k9UUTVu1q6+YHZha34NbYh40y70H3kTsa6mc=; b=djbPt6Xo9zJywBkbFFepTpwQAy7nXQlxnlgOAxtslc4VnSzK9QGak9PPcdeuSmDvX3 fV+AGmk0HYQ70YSYZfuDJbGRNH3qjDsILoEbJyrsPJKeg6xE2tqoalrPlApy+7Ttp7YB RUPBSLwRI0fEVqc5ak11rEL/KAAwN98nlSZHbVCPczQfwKg49zwBJLGaPsLZFopSLO64 Uey3idZe9Rlhjy3BRvwtE1I/Y57JyIJhdg2HoZiZrPj/vBC2Zi0ZYTfRb1W61Q3b+h7C zfbdu7l0wYymjU9ob9AJ/fLJyB+eefuLWz6At1tSUrzXEm/oB6rJThrE7UIZNN6f7Or5 xP0Q== X-Forwarded-Encrypted: i=1; AJvYcCXoROJyajvwp40c9xCFoN6FiMHZ5j6A3znPDzgVYzdRbsHkD+qmFfxWXRie5y8Ze9wrXvM=@vger.kernel.org X-Gm-Message-State: AOJu0YyNFYZpOkzd0hYJWWCX6wv596BlIeAbjLgWNWFi4urduf1D864J Z99kqOUyl9ROnIwFmdlMjBZSekp/EnYw4XgVkCLglZU0LKY2PaxTVo1DK2J87j3HGVyXgpqyzw5 w+Q== X-Google-Smtp-Source: AGHT+IH2c0dkWdxSoUH/P+c/MHYnOos+sGeNLUsxpV+Tj6PKAdhff85ZIO3F/CXDLvmSmNqvrbsH0zqif9M= X-Received: from pjbsq11.prod.google.com ([2002:a17:90b:530b:b0:2f7:ff61:48e7]) (user=seanjc job=prod-delivery.src-stubby-dispatcher) by 2002:a17:90b:2d8c:b0:2ea:2a8d:dd2a with SMTP id 98e67ed59e1d1-2fa242e6928mr22387097a91.27.1739227301444; Mon, 10 Feb 2025 14:41:41 -0800 (PST) Date: Mon, 10 Feb 2025 14:41:40 -0800 In-Reply-To: Precedence: bulk X-Mailing-List: kvm@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 References: <20250207030640.1585-1-yan.y.zhao@intel.com> <20250207030931.1902-1-yan.y.zhao@intel.com> Message-ID: Subject: Re: [PATCH 4/4] KVM: x86/mmu: Free obsolete roots when pre-faulting SPTEs From: Sean Christopherson To: Yan Zhao Cc: pbonzini@redhat.com, rick.p.edgecombe@intel.com, linux-kernel@vger.kernel.org, kvm@vger.kernel.org Content-Type: text/plain; charset="us-ascii" On Sat, Feb 08, 2025, Yan Zhao wrote: > On Fri, Feb 07, 2025 at 07:12:04AM -0800, Sean Christopherson wrote: > > On Fri, Feb 07, 2025, Yan Zhao wrote: > > > Always free obsolete roots when pre-faulting SPTEs in case it's called > > > after a root is invalidated (e.g., by memslot removal) but before any > > > vcpu_enter_guest() processing of KVM_REQ_MMU_FREE_OBSOLETE_ROOTS. > > > > > > Lack of kvm_mmu_free_obsolete_roots() in this scenario can lead to > > > kvm_mmu_reload() failing to load a new root if the current root hpa is an > > > obsolete root (which is not INVALID_PAGE). Consequently, > > > kvm_arch_vcpu_pre_fault_memory() will retry infinitely due to the checking > > > of is_page_fault_stale(). > > > > > > It's safe to call kvm_mmu_free_obsolete_roots() even if there are no > > > obsolete roots or if it's called a second time when vcpu_enter_guest() > > > later processes KVM_REQ_MMU_FREE_OBSOLETE_ROOTS. This is because > > > kvm_mmu_free_obsolete_roots() sets an obsolete root to INVALID_PAGE and > > > will do nothing to an INVALID_PAGE. > > > > Why is userspace changing memslots while prefaulting? > It currently only exists in the kvm selftest (written by myself...) > Not sure if there's any real use case like this. It's decidedly odd. I asked, because maybe there's a way we can disallow the scenario. Doing that without making things more complex than simply handling obsolete roots is probably a fool's errand though. > > > Signed-off-by: Yan Zhao > > > --- > > > arch/x86/kvm/mmu/mmu.c | 5 +++++ > > > 1 file changed, 5 insertions(+) > > > > > > diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c > > > index 47fd3712afe6..72f68458049a 100644 > > > --- a/arch/x86/kvm/mmu/mmu.c > > > +++ b/arch/x86/kvm/mmu/mmu.c > > > @@ -4740,7 +4740,12 @@ long kvm_arch_vcpu_pre_fault_memory(struct kvm_vcpu *vcpu, > > > /* > > > * reload is efficient when called repeatedly, so we can do it on > > > * every iteration. > > > + * Before reload, free obsolete roots in case the prefault is called > > > + * after a root is invalidated (e.g., by memslot removal) but > > > + * before any vcpu_enter_guest() processing of > > > + * KVM_REQ_MMU_FREE_OBSOLETE_ROOTS. > > > */ > > > + kvm_mmu_free_obsolete_roots(vcpu); > > > r = kvm_mmu_reload(vcpu); > > > if (r) > > > return r; > > > > I would prefer to do check for obsolete roots in kvm_mmu_reload() itself, but > Yes, it's better! > I previously considered doing in this way, but I was afraid to introduce > overhead (the extra compare) to kvm_mmu_reload(), which is called quite > frequently. > > But maybe we can remove the check in vcpu_enter_guest() to reduce the overhead? > > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c > index b2d9a16fd4d3..6a1f2780a094 100644 > --- a/arch/x86/kvm/x86.c > +++ b/arch/x86/kvm/x86.c > @@ -10731,8 +10731,6 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu) > goto out; > } > } > - if (kvm_check_request(KVM_REQ_MMU_FREE_OBSOLETE_ROOTS, vcpu)) > - kvm_mmu_free_obsolete_roots(vcpu); > if (kvm_check_request(KVM_REQ_MIGRATE_TIMER, vcpu)) > __kvm_migrate_timers(vcpu); > if (kvm_check_request(KVM_REQ_MASTERCLOCK_UPDATE, vcpu)) > > > keep the main kvm_check_request() so that the common case handles the resulting > > TLB flush without having to loop back around in vcpu_enter_guest(). > Hmm, I'm a little confused. > What's is the resulting TLB flush? For the common case where KVM_REQ_MMU_FREE_OBSOLETE_ROOTS is pending before vcpu_enter_guest, kvm_mmu_free_obsolete_roots() may trigger KVM_REQ_TLB_FLUSH via kvm_mmu_commit_zap_page(). Processing KVM_REQ_MMU_FREE_OBSOLETE_ROOTS before KVM_REQ_TLB_FLUSH means vcpu_enter_guest() doesn't have to "abort" and redo the whole loop (the newly pending request won't be detected until kvm_vcpu_exit_request(), which isn't that late in the entry sequence, but there is a decent amount of work that needs to be undone). On the other hand, the cost of kvm_check_request(), especially a check that's guarded by kvm_request_pending(), is negligible. That said, obsolete roots shouldn't actually require a TLB flush. E.g. the TDP MMU hasn't flushed invalid roots since commit fcdffe97f80e ("KVM: x86/mmu: Don't do TLB flush when zappings SPTEs in invalid roots"). I'd have to think more about whether or not that's safe/correct for the shadow MMU though. For this case, I think it makes sense to just add the check in kvm_mmu_reload().