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Wed, 04 Sep 2024 15:23:01 -0700 (PDT) Date: Wed, 4 Sep 2024 15:23:00 -0700 In-Reply-To: Mime-Version: 1.0 References: <20240903191033.28365-1-Ashish.Kalra@amd.com> <25ca73c9-e4ba-4a95-82c8-0d6cf8d0ff78@redhat.com> <14b0bc83-f645-408f-b8af-13f49fe6155d@amd.com> <20240904195408.wfaukcphpw5iwjcg@amd.com> Message-ID: Subject: Re: [PATCH v2] x86/sev: Fix host kdump support for SNP From: Sean Christopherson To: Ashish Kalra Cc: Michael Roth , Paolo Bonzini , dave.hansen@linux.intel.com, tglx@linutronix.de, mingo@redhat.com, bp@alien8.de, x86@kernel.org, hpa@zytor.com, peterz@infradead.org, linux-kernel@vger.kernel.org, kvm@vger.kernel.org, thomas.lendacky@amd.com, kexec@lists.infradead.org, linux-coco@lists.linux.dev X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20240904_152302_898410_E903DA43 X-CRM114-Status: GOOD ( 25.53 ) X-BeenThere: kexec@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Type: text/plain; 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Wed, 04 Sep 2024 15:23:01 -0700 (PDT) Date: Wed, 4 Sep 2024 15:23:00 -0700 In-Reply-To: Precedence: bulk X-Mailing-List: kvm@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 References: <20240903191033.28365-1-Ashish.Kalra@amd.com> <25ca73c9-e4ba-4a95-82c8-0d6cf8d0ff78@redhat.com> <14b0bc83-f645-408f-b8af-13f49fe6155d@amd.com> <20240904195408.wfaukcphpw5iwjcg@amd.com> Message-ID: Subject: Re: [PATCH v2] x86/sev: Fix host kdump support for SNP From: Sean Christopherson To: Ashish Kalra Cc: Michael Roth , Paolo Bonzini , dave.hansen@linux.intel.com, tglx@linutronix.de, mingo@redhat.com, bp@alien8.de, x86@kernel.org, hpa@zytor.com, peterz@infradead.org, linux-kernel@vger.kernel.org, kvm@vger.kernel.org, thomas.lendacky@amd.com, kexec@lists.infradead.org, linux-coco@lists.linux.dev Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: quoted-printable On Wed, Sep 04, 2024, Ashish Kalra wrote: > On 9/4/2024 2:54 PM, Michael Roth wrote: > > - Sean inquired about making the target kdump kernel more agnostic to > > whether or not SNP_SHUTDOWN was done properly, since that might > > allow for capturing state even for edge cases where we can't go > > through the normal cleanup path. I mentioned we'd tried this to som= e > > degree but hit issues with the IOMMU, and when working around that > > there was another issue but I don't quite recall the specifics. > > Can you post a quick recap of what the issues are with that approac= h > > so we can determine whether or not this is still an option? >=20 > Yes, i believe without SNP_SHUTDOWN, early_enable_iommus() configure the > IOMMUs into an IRQ remapping configuration causing the crash in > io_apic.c::check_timer(). >=20 > It looks like in this case, we enable IRQ remapping configuration *earlie= r* > than when it needs to be enabled and which causes the panic as indicated: >=20 > EMERGENCY [=C2=A0=C2=A0=C2=A0 1.376701] Kernel panic - not syncing: timer= doesn't work > through Interrupt-remapped IO-APIC I assume the problem is that IOMMU setup fails in the kdump kernel, not tha= t it does the setup earlier. That's that part I want to understand. Based on the SNP ABI: The firmware initializes the IOMMU to perform RMP enforcement. The firmwa= re also transitions the event log, PPR log, and completion wait buffers of the IO= MMU to an RMP page state that is read only to the hypervisor and cannot be assig= ned to guests. and commit f366a8dac1b8 ("iommu/amd: Clean up RMP entries for IOMMU pages d= uring SNP shutdown"), my understanding is that the pages used for the IOMMU logs = are forced to read-only for the IOMMU, and so attempting to access those pages = in the kdump kernel will result in an RMP #PF. That's quite unfortunate, as it means my idea of eating RMP #PFs doesn't re= ally work, because that idea is based on the assumption that only guest private = memory would generate unexpected RMP #PFs :-( > Next, we tried with amd_iommu=3Doff, with that we don't get the irq remap= ping > panic during crashkernel boot, but boot still hangs before starting kdump > tools. >=20 > So eventually we discovered that irqremapping is required for x2apic and = with > amd_iommu=3Doff we don't enable irqremapping at all. Yeah, that makes sense, as does failing to boot if the system isn't configu= red properly, i.e. can't send interrupts to all CPUs.