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Tue, 18 Nov 2025 15:31:24 -0800 (PST) Date: Tue, 18 Nov 2025 15:31:22 -0800 In-Reply-To: <20251028002824.1470939-1-rick.p.edgecombe@intel.com> X-Mailing-List: linuxppc-dev@lists.ozlabs.org List-Id: List-Help: List-Owner: List-Post: List-Archive: , List-Subscribe: , , List-Unsubscribe: Precedence: list Mime-Version: 1.0 References: <20251028002824.1470939-1-rick.p.edgecombe@intel.com> Message-ID: Subject: Re: [PATCH] KVM: TDX: Take MMU lock around tdh_vp_init() From: Sean Christopherson To: Rick Edgecombe Cc: ackerleytng@google.com, anup@brainfault.org, aou@eecs.berkeley.edu, binbin.wu@linux.intel.com, borntraeger@linux.ibm.com, chenhuacai@kernel.org, frankja@linux.ibm.com, imbrenda@linux.ibm.com, ira.weiny@intel.com, kai.huang@intel.com, kas@kernel.org, kvm-riscv@lists.infradead.org, kvm@vger.kernel.org, kvmarm@lists.linux.dev, linux-arm-kernel@lists.infradead.org, linux-coco@lists.linux.dev, linux-kernel@vger.kernel.org, linux-mips@vger.kernel.org, linux-riscv@lists.infradead.org, linuxppc-dev@lists.ozlabs.org, loongarch@lists.linux.dev, maddy@linux.ibm.com, maobibo@loongson.cn, maz@kernel.org, michael.roth@amd.com, oliver.upton@linux.dev, palmer@dabbelt.com, pbonzini@redhat.com, pjw@kernel.org, vannapurve@google.com, x86@kernel.org, yan.y.zhao@intel.com, zhaotianrui@loongson.cn Content-Type: text/plain; charset="us-ascii" On Mon, Oct 27, 2025, Rick Edgecombe wrote: > Take MMU lock around tdh_vp_init() in KVM_TDX_INIT_VCPU to prevent > meeting contention during retries in some no-fail MMU paths. > > The TDX module takes various try-locks internally, which can cause > SEAMCALLs to return an error code when contention is met. Dealing with > an error in some of the MMU paths that make SEAMCALLs is not straight > forward, so KVM takes steps to ensure that these will meet no contention > during a single BUSY error retry. The whole scheme relies on KVM to take > appropriate steps to avoid making any SEAMCALLs that could contend while > the retry is happening. > > Unfortunately, there is a case where contention could be met if userspace > does something unusual. Specifically, hole punching a gmem fd while > initializing the TD vCPU. The impact would be triggering a KVM_BUG_ON(). > > The resource being contended is called the "TDR resource" in TDX docs > parlance. The tdh_vp_init() can take this resource as exclusive if the > 'version' passed is 1, which happens to be version the kernel passes. The > various MMU operations (tdh_mem_range_block(), tdh_mem_track() and > tdh_mem_page_remove()) take it as shared. > > There isn't a KVM lock that maps conceptually and in a lock order friendly > way to the TDR lock. So to minimize infrastructure, just take MMU lock > around tdh_vp_init(). This makes the operations we care about mutually > exclusive. Since the other operations are under a write mmu_lock, the code > could just take the lock for read, however this is weirdly inverted from > the actual underlying resource being contended. Since this is covering an > edge case that shouldn't be hit in normal usage, be a little less weird > and take the mmu_lock for write around the call. > > Fixes: 02ab57707bdb ("KVM: TDX: Implement hooks to propagate changes of TDP MMU mirror page table") > Reported-by: Yan Zhao > Suggested-by: Yan Zhao > Signed-off-by: Rick Edgecombe > --- > Hi, > > It was indeed awkward, as Sean must have sniffed. But seems ok enough to > close the issue. > > Yan, can you give it a look? > > Posted here, but applies on top of this series. In the future, please don't post in-reply-to, as it mucks up my b4 workflow. Applied to kvm-x86 tdx, with a more verbose comment as suggested by Binbin. [1/1] KVM: TDX: Take MMU lock around tdh_vp_init() https://github.com/kvm-x86/linux/commit/9a89894f30d5