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dkim=none (message not signed) header.d=none;lists.xenproject.org; dmarc=none action=none header.from=oracle.com; Subject: Re: [PATCH v2 3/4] x86: Allow non-faulting accesses to non-emulated MSRs if policy permits this To: Jan Beulich Cc: iwj@xenproject.org, wl@xen.org, anthony.perard@citrix.com, andrew.cooper3@citrix.com, roger.pau@citrix.com, jun.nakajima@intel.com, kevin.tian@intel.com, xen-devel@lists.xenproject.org References: <1611182952-9941-1-git-send-email-boris.ostrovsky@oracle.com> <1611182952-9941-4-git-send-email-boris.ostrovsky@oracle.com> From: Boris Ostrovsky Message-ID: Date: Fri, 22 Jan 2021 14:52:59 -0500 User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:78.0) Gecko/20100101 Thunderbird/78.6.1 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit X-Originating-IP: [138.3.200.16] X-ClientProxiedBy: BY5PR16CA0033.namprd16.prod.outlook.com (2603:10b6:a03:1a0::46) To BYAPR10MB3288.namprd10.prod.outlook.com (2603:10b6:a03:156::21) MIME-Version: 1.0 X-MS-Exchange-MessageSentRepresentingType: 1 X-MS-PublicTrafficType: Email X-MS-Office365-Filtering-Correlation-Id: 1d084cf1-2404-41f3-0259-08d8bf0f53df X-MS-TrafficTypeDiagnostic: BYAPR10MB3320: X-Microsoft-Antispam-PRVS: X-MS-Oob-TLC-OOBClassifiers: OLM:8273; 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X-MS-Exchange-AntiSpam-MessageData: =?utf-8?B?SkR4SzJLRHlGWnZhd0UyM0FkeHNLZ1dDMC9rY0dlTTc1dXlqVlVVWEtwUENZ?= =?utf-8?B?UnZ4ZTJtZzZDbWRjQnNadytWZnNsOXJINllPOHRlbjM3ME53WDAvMlY2MUl4?= =?utf-8?B?b2t6K1hjUFY0K2QwZ2UzM21XMGROV25lUElSRkkrekZFeDFia1BwU3pqR3FE?= =?utf-8?B?dGc4aElVL0FIU2NnZnFZSWhZZCtYaHNJMHVsdzVVZk5Ld1NVYnZzd3VLaDg4?= =?utf-8?B?Rzc5LzNZdWF0MkdCUEp0aWVvQXFZanJ5eWZtNU55OEo1UTRScWtlaDRYZjdI?= =?utf-8?B?cHBOTXdDSE9Qd1ZVejN6d2xGWHVXVVJELzM0NkU3M3AwVmpsZDRpWlJQYXFy?= =?utf-8?B?Q2p2S0xEWlVkbmYrZkp0V01FeDlDc0x6bjhPc0Q1U1NOSGVSUFZKK2dvaUFl?= =?utf-8?B?cFhXN1pINnRuSEZDTkQyc1p3aFQ2MkVkZEtsUGpNWHdnUXFCNVdpRDVPZXJZ?= =?utf-8?B?R2dyZ2tnNm1rd3dEYWV3UTlBUm9zblBCbFpJTW8xcmx5T1h2L3dkUUtxaVZ5?= =?utf-8?B?L3NLUW1pOFk2UHBnZXZRWnZhUWtDbG44cDhERml3eW1FenFyQWwxTUpGbVlV?= =?utf-8?B?dHRDZXBSRk96MEU0MmUzbTl2aWNMUmh5cFY2eXdJZk96ZWxOTHVLVm51VFJu?= =?utf-8?B?dUFxRzdrRjliLzFYMjIrSGR3ckM2VU9zcy9nQTFTaGpWWlJpZ3pPK2NjRjI1?= =?utf-8?B?Z2grZlBtdk9mVVJRWjFWSnlCdkhrR29zVGRRYythZFpTQkRxOHZLVVd6TTJ2?= =?utf-8?B?dk0rdUNjVGJhYkh6N0pJamxlSE8zU2srNlJIVEw1eHhwaVBtSklxY0lGNmYw?= =?utf-8?B?VUhXdXFUYXIzYU9oS3gvYWtMY0JFeVh0cE1MU0w0UVpHcnQxRmZ5dC96ZzBD?= =?utf-8?B?SFI2S2crWTVDTzMxZldZZHJveHFVb25MVVVDWitNU3RqcHN3RGpwb2NTSVpJ?= =?utf-8?B?MzFwSHRxOHpKb0lvUVU2blZzVi85aVd4aXdHNWdmWktnLy9tRFMzMDdaaFZv?= =?utf-8?B?a2ZKNllwUDZLNGVnTWxEMk90d1phYVpyY0o5WXcrY29JL0ovZWs2YXByT1Br?= =?utf-8?B?OTdXajdkOWl5TVBMYkpJOXZSSmhPZmxUZzRZbUhDbVVWazVqSHkvZlpWc2h3?= =?utf-8?B?bHVQYTZ2MDhJbjdRYWdTeThSYURmazRmYWdwNXdQMDE5TDBMNDJtUFNYZ2lY?= =?utf-8?B?VDk3eWFGR1hoOXkxSzFrbU52c24wWlk4Vk1XVVRTZU1QZnpzaDV4OGRzVTVn?= =?utf-8?B?bTNXNTBwak5tZFJ6SkdFR25SekNkdGxEbEIrRjFOQ0NRd2ZrYXlraWtNcEF6?= =?utf-8?Q?ydtsb8SwPr1ccXd4SDJFBPK091s19WOeBy?= X-OriginatorOrg: oracle.com X-MS-Exchange-CrossTenant-Network-Message-Id: 1d084cf1-2404-41f3-0259-08d8bf0f53df X-MS-Exchange-CrossTenant-AuthSource: BYAPR10MB3288.namprd10.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Internal X-MS-Exchange-CrossTenant-OriginalArrivalTime: 22 Jan 2021 19:53:02.7023 (UTC) X-MS-Exchange-CrossTenant-FromEntityHeader: Hosted X-MS-Exchange-CrossTenant-Id: 4e2c6054-71cb-48f1-bd6c-3a9705aca71b X-MS-Exchange-CrossTenant-MailboxType: HOSTED X-MS-Exchange-CrossTenant-UserPrincipalName: mtkrYq+Gm826XoMJMn4Peler+hRxwC0QL4oh1CbOTdPc0wec/QPCc4zodgLDuedHuVSWrHvRojijNA/tOsIkvXHSRu9rJHuDGnewoq0SQa4= X-MS-Exchange-Transport-CrossTenantHeadersStamped: BYAPR10MB3320 X-Proofpoint-Virus-Version: vendor=nai engine=6200 definitions=9872 signatures=668683 X-Proofpoint-Spam-Details: rule=notspam policy=default score=0 mlxscore=0 spamscore=0 suspectscore=0 adultscore=0 mlxlogscore=999 bulkscore=0 malwarescore=0 phishscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2009150000 definitions=main-2101220101 X-Proofpoint-Virus-Version: vendor=nai engine=6200 definitions=9872 signatures=668683 X-Proofpoint-Spam-Details: rule=notspam policy=default score=0 mlxlogscore=999 mlxscore=0 suspectscore=0 lowpriorityscore=0 bulkscore=0 adultscore=0 spamscore=0 phishscore=0 priorityscore=1501 impostorscore=0 malwarescore=0 clxscore=1015 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2009150000 definitions=main-2101220101 On 1/22/21 7:51 AM, Jan Beulich wrote: > On 20.01.2021 23:49, Boris Ostrovsky wrote: > >> + >> + /* >> + * Accesses to unimplemented MSRs as part of emulation of instructions >> + * other than guest's RDMSR/WRMSR should never succeed. >> + */ >> + if ( !is_guest_msr_access ) >> + ignore_msrs = MSR_UNHANDLED_NEVER; > > Wouldn't you better "return true" here? Such accesses also > shouldn't be logged imo (albeit I agree that's a change from > current behavior). Yes, that's why I didn't return here. We will be here in !is_guest_msr_access case most likely due to a bug in the emulator so I think we do want to see the error logged. > >> + if ( unlikely(ignore_msrs != MSR_UNHANDLED_NEVER) ) >> + *val = 0; > > I don't understand the conditional here, even more so with > the respective changelog entry. In any event you don't > want to clobber the value ahead of ... > >> + if ( likely(ignore_msrs != MSR_UNHANDLED_SILENT) ) >> + { >> + if ( is_write ) >> + gdprintk(XENLOG_WARNING, "WRMSR 0x%08x val 0x%016"PRIx64 >> + " unimplemented\n", msr, *val); > > ... logging it. True. I dropped !is_write from v1 without considering this. As far as the conditional --- dropping it too would be a behavior change. > >> --- a/xen/arch/x86/x86_emulate/x86_emulate.h >> +++ b/xen/arch/x86/x86_emulate/x86_emulate.h >> @@ -850,4 +850,10 @@ static inline void x86_emul_reset_event(struct x86_emulate_ctxt *ctxt) >> ctxt->event = (struct x86_event){}; >> } >> >> +static inline bool x86_emul_guest_msr_access(struct x86_emulate_ctxt *ctxt) > > The parameter wants to be pointer-to-const. In addition I wonder > whether this wouldn't better be a sibling to > x86_insn_is_cr_access() (without a "state" parameter, which > would be unused and unavailable to the callers), which may end > up finding further uses down the road. "Sibling" in terms of name (yes, it would be) or something else? > >> +{ >> + return ctxt->opcode == X86EMUL_OPC(0x0f, 0x32) || /* RDMSR */ >> + ctxt->opcode == X86EMUL_OPC(0x0f, 0x30); /* WRMSR */ >> +} > > Personally I'd prefer if this was a single comparison: > > return (ctxt->opcode | 2) == X86EMUL_OPC(0x0f, 0x32); > > But maybe nowadays' compilers are capable of this > transformation? Here is what I've got (not an inline but shouldn't make much difference I'd think) ffff82d040385960 : # your code ffff82d040385960: 8b 47 2c mov 0x2c(%rdi),%eax ffff82d040385963: 83 e0 fd and $0xfffffffd,%eax ffff82d040385966: 3d 30 00 0f 00 cmp $0xf0030,%eax ffff82d04038596b: 0f 94 c0 sete %al ffff82d04038596e: c3 retq ffff82d04038596f : # my code ffff82d04038596f: 8b 47 2c mov 0x2c(%rdi),%eax ffff82d040385972: 83 c8 02 or $0x2,%eax ffff82d040385975: 3d 32 00 0f 00 cmp $0xf0032,%eax ffff82d04038597a: 0f 94 c0 sete %al ffff82d04038597d: c3 retq So it's a wash in terms of generated code. > > I notice you use this function only from PV priv-op emulation. > What about the call paths through hvmemul_{read,write}_msr()? > (It's also questionable whether the write paths need this - > the only MSR written outside of WRMSR emulation is > MSR_SHADOW_GS_BASE, which can't possibly reach the "unhandled" > logic anywhere. But maybe better to be future proof here in > case new MSR writes appear in the emulator, down the road.) Won't we end up in hvm_funcs.msr_write_intercept ops which do call it? -boris