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AJvYcCUpyg0CgLflcCC4P7OWmiiNhMq2TxDKCBgIQWiQzbe0byYJqMVDoLlk6ucmpTprKfs5RtVrYNVGO3hHWt4UWDse5fz9 X-Gm-Message-State: AOJu0YzpZeHtV7EGuUxOO3N+CJED4zILkdsNqxroqKUqdBV11h2fqtiK m1gvkdG7HzBqCg/Br7LNAPvcBhFO2PTX88zRcuBeqNdUE3FaTko9ZZ65iyrNXR2eRdzV8WnPgtq bDg== X-Google-Smtp-Source: AGHT+IHSFYB5Ovi0b4K5jyx+4UuWpvG3spx7cgwCfDIbLmGdSagtoqec1hfiGwAc/ZwSddi8gX2/4CqCx5Y= X-Received: from zagreus.c.googlers.com ([fda3:e722:ac3:cc00:7f:e700:c0a8:5c37]) (user=seanjc job=sendgmr) by 2002:a17:902:da8c:b0:1e5:e651:6099 with SMTP id j12-20020a170902da8c00b001e5e6516099mr727874plx.9.1713188977177; Mon, 15 Apr 2024 06:49:37 -0700 (PDT) Date: Mon, 15 Apr 2024 06:49:35 -0700 In-Reply-To: <20240413004031.GQ3039520@ls.amr.corp.intel.com> Precedence: bulk X-Mailing-List: kvm@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 References: <0c3efffa-8dd5-4231-8e90-e0241f058a20@intel.com> <20240412214201.GO3039520@ls.amr.corp.intel.com> <20240413004031.GQ3039520@ls.amr.corp.intel.com> Message-ID: Subject: Re: [PATCH v19 087/130] KVM: TDX: handle vcpu migration over logical processor From: Sean Christopherson To: Isaku Yamahata Cc: Reinette Chatre , kvm@vger.kernel.org, linux-kernel@vger.kernel.org, isaku.yamahata@gmail.com, Paolo Bonzini , erdemaktas@google.com, Sagi Shahar , Kai Huang , chen.bo@intel.com, hang.yuan@intel.com, tina.zhang@intel.com, isaku.yamahata@linux.intel.com Content-Type: text/plain; charset="us-ascii" On Fri, Apr 12, 2024, Isaku Yamahata wrote: > On Fri, Apr 12, 2024 at 03:46:05PM -0700, > Sean Christopherson wrote: > > > On Fri, Apr 12, 2024, Isaku Yamahata wrote: > > > On Fri, Apr 12, 2024 at 09:15:29AM -0700, Reinette Chatre wrote: > > > > > +void tdx_mmu_release_hkid(struct kvm *kvm) > > > > > +{ > > > > > + while (__tdx_mmu_release_hkid(kvm) == -EBUSY) > > > > > + ; > > > > > } > > > > > > > > As I understand, __tdx_mmu_release_hkid() returns -EBUSY > > > > after TDH.VP.FLUSH has been sent for every vCPU followed by > > > > TDH.MNG.VPFLUSHDONE, which returns TDX_FLUSHVP_NOT_DONE. > > > > > > > > Considering earlier comment that a retry of TDH.VP.FLUSH is not > > > > needed, why is this while() loop here that sends the > > > > TDH.VP.FLUSH again to all vCPUs instead of just a loop within > > > > __tdx_mmu_release_hkid() to _just_ resend TDH.MNG.VPFLUSHDONE? > > > > > > > > Could it be possible for a vCPU to appear during this time, thus > > > > be missed in one TDH.VP.FLUSH cycle, to require a new cycle of > > > > TDH.VP.FLUSH? > > > > > > Yes. There is a race between closing KVM vCPU fd and MMU notifier release hook. > > > When KVM vCPU fd is closed, vCPU context can be loaded again. > > > > But why is _loading_ a vCPU context problematic? > > It's nothing problematic. It becomes a bit harder to understand why > tdx_mmu_release_hkid() issues IPI on each loop. I think it's reasonable > to make the normal path easy and to complicate/penalize the destruction path. > Probably I should've added comment on the function. By "problematic", I meant, why can that result in a "missed in one TDH.VP.FLUSH cycle"? AFAICT, loading a vCPU shouldn't cause that vCPU to be associated from the TDX module's perspective, and thus shouldn't trigger TDX_FLUSHVP_NOT_DONE. I.e. looping should be unnecessary, no?