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Received-SPF: Pass (protection.outlook.com: domain of amd.com designates 165.204.84.17 as permitted sender) receiver=protection.outlook.com; client-ip=165.204.84.17; helo=SATLEXMB04.amd.com; pr=C Received: from SATLEXMB04.amd.com (165.204.84.17) by DS3PEPF000099D9.mail.protection.outlook.com (10.167.17.10) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256) id 15.20.6838.14 via Frontend Transport; Tue, 3 Oct 2023 07:31:50 +0000 Received: from aiemdee.2.ozlabs.ru (10.180.168.240) by SATLEXMB04.amd.com (10.181.40.145) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256) id 15.1.2507.27; Tue, 3 Oct 2023 02:31:45 -0500 From: Alexey Kardashevskiy To: CC: , Ingo Molnar , "Borislav Petkov" , Tom Lendacky , "Alexey Kardashevskiy" Subject: [PATCH kernel v3] x86/compressed/64: reduce #VC nesting for intercepted CPUID for SEV-SNP guest Date: Tue, 3 Oct 2023 18:31:23 +1100 Message-ID: <20231003073123.1763564-1-aik@amd.com> X-Mailer: git-send-email 2.41.0 MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable X-Originating-IP: [10.180.168.240] X-ClientProxiedBy: SATLEXMB03.amd.com (10.181.40.144) To SATLEXMB04.amd.com (10.181.40.145) X-EOPAttributedMessage: 0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: DS3PEPF000099D9:EE_|DS7PR12MB6240:EE_ X-MS-Office365-Filtering-Correlation-Id: f4753995-f295-485e-90b3-08dbc3e2ced0 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0; 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X-OriginatorOrg: amd.com X-MS-Exchange-CrossTenant-OriginalArrivalTime: 03 Oct 2023 07:31:50.2076 (UTC) X-MS-Exchange-CrossTenant-Network-Message-Id: f4753995-f295-485e-90b3-08dbc3e2ced0 X-MS-Exchange-CrossTenant-Id: 3dd8961f-e488-4e60-8e11-a82d994e183d X-MS-Exchange-CrossTenant-OriginalAttributedTenantConnectingIp: TenantId=3dd8961f-e488-4e60-8e11-a82d994e183d;Ip=[165.204.84.17];Helo=[SATLEXMB04.amd.com] X-MS-Exchange-CrossTenant-AuthSource: DS3PEPF000099D9.namprd04.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Anonymous X-MS-Exchange-CrossTenant-FromEntityHeader: HybridOnPrem X-MS-Exchange-Transport-CrossTenantHeadersStamped: DS7PR12MB6240 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Type: text/plain; charset="utf-8" For certain intercepts an SNP guest uses the GHCB protocol to talk to the hypervisor from the #VC handler. The protocol requires a shared page so there is one per vCPU. In case NMI arrives in a middle of #VC or the NMI handler triggers a #VC, there is another "backup" GHCB page which stores the content of the first one while SVM_VMGEXIT_NMI_COMPLETE is sent. The vc_raw_handle_exception() handler manages main and backup GHCB pages via __sev_get_ghcb/__sev_put_ghcb. This works fine for #VC and occasional NMIs but not so fine when the #VC handler causes intercept + another #VC. If NMI arrives during the second #VC, there are no more pages for SVM_VMGEXIT_NMI_COMPLETE. The problem place is the #VC CPUID handler which reads an MSR which triggers another #VC and if "perf" was running, panic happens: Kernel panic - not syncing: Unable to handle #VC exception! GHCB and Backup= GHCB are already in use Add a helper similar to native_read_msr_safe() for making a direct hypercall in the SEV-ES environment. Use the new helper instead of the raw "rdmsr" to avoid the extra #VC event. Fixes: ee0bfa08a345 ("x86/compressed/64: Add support for SEV-SNP CPUID tabl= e in #VC handlers") Signed-off-by: Alexey Kardashevskiy Reviewed-by: Ingo Molnar --- Based on: https://git.kernel.org/pub/scm/linux/kernel/git/bp/bp.git/log/?h=3Dtip-x86-= urgent which top at the time was: 62d5e970d022 "x86/sev: Change npages to unsigned long in snp_accept_memory(= )" --- Changes: v3: * made it a function, mimic native_read_msr_safe() which 1) returns value 2= ) returns an error * removed debug backtraces the commit log as these were added for debugging= and never appear with actual kernels v2: * de-uglify by defining rdmsr_safe_GHCB() --- arch/x86/kernel/sev-shared.c | 27 +++++++++++++++++--- 1 file changed, 23 insertions(+), 4 deletions(-) diff --git a/arch/x86/kernel/sev-shared.c b/arch/x86/kernel/sev-shared.c index dcf325b7b022..494d92a71986 100644 --- a/arch/x86/kernel/sev-shared.c +++ b/arch/x86/kernel/sev-shared.c @@ -241,6 +241,25 @@ static enum es_result sev_es_ghcb_hv_call(struct ghcb = *ghcb, return verify_exception_info(ghcb, ctxt); } =20 + +/* Paravirt SEV-ES rdmsr which avoids extra #VC event */ +static unsigned long long ghcb_prot_read_msr(unsigned int msr, struct ghcb= *ghcb, + struct es_em_ctxt *ctxt, int *err) +{ + unsigned long long ret =3D 0; + + ghcb_set_rcx(ghcb, msr); + + *err =3D sev_es_ghcb_hv_call(ghcb, ctxt, SVM_EXIT_MSR, 0, 0); + if (*err =3D=3D ES_OK) + ret =3D (ghcb->save.rdx << 32) | ghcb->save.rax; + + /* Invalidate qwords for likely another following GHCB call */ + vc_ghcb_invalidate(ghcb); + + return ret; +} + static int __sev_cpuid_hv(u32 fn, int reg_idx, u32 *reg) { u64 val; @@ -477,11 +496,11 @@ static int snp_cpuid_postprocess(struct ghcb *ghcb, s= truct es_em_ctxt *ctxt, if (leaf->subfn =3D=3D 1) { /* Get XSS value if XSAVES is enabled. */ if (leaf->eax & BIT(3)) { - unsigned long lo, hi; + int err =3D 0; =20 - asm volatile("rdmsr" : "=3Da" (lo), "=3Dd" (hi) - : "c" (MSR_IA32_XSS)); - xss =3D (hi << 32) | lo; + xss =3D ghcb_prot_read_msr(MSR_IA32_XSS, ghcb, ctxt, &err); + if (err !=3D ES_OK) + return -EINVAL; } =20 /* --=20 2.41.0