Received-SPF: pass (google.com: domain of linux-kernel+bounces-202894-linux.lists.archive=gmail.com@vger.kernel.org designates 147.75.48.161 as permitted sender) client-ip=147.75.48.161;
Message-ID: <59381f4f-94de-4933-9dbd-f0fbdc5d5e4a@amd.com>
Date: Wed, 5 Jun 2024 21:44:13 +0530
User-Agent: Mozilla Thunderbird
From: Ravi Bangoria <ravi.bangoria@amd.com>
Subject: Re: [PATCH 3/3] KVM SVM: Add Bus Lock Detect support
To: Sean Christopherson <seanjc@google.com>
Cc: tglx@linutronix.de, mingo@redhat.com, bp@alien8.de,
 dave.hansen@linux.intel.com, pbonzini@redhat.com, thomas.lendacky@amd.com,
 hpa@zytor.com, rmk+kernel@armlinux.org.uk, peterz@infradead.org,
 james.morse@arm.com, lukas.bulwahn@gmail.com, arjan@linux.intel.com,
 j.granados@samsung.com, sibs@chinatelecom.cn, nik.borisov@suse.com,
 michael.roth@amd.com, nikunj.dadhania@amd.com, babu.moger@amd.com,
 x86@kernel.org, kvm@vger.kernel.org, linux-kernel@vger.kernel.org,
 santosh.shukla@amd.com, ananth.narayan@amd.com, sandipan.das@amd.com,
 ravi.bangoria@amd.com
References: <20240429060643.211-1-ravi.bangoria@amd.com>
 <20240429060643.211-4-ravi.bangoria@amd.com> <Zl5jqwWO4FyawPHG@google.com>
 <e1c29dd4-2eb9-44fe-abf2-f5ca0e84e2a6@amd.com> <ZmB_hl7coZ_8KA8Q@google.com>
Content-Language: en-US
In-Reply-To: <ZmB_hl7coZ_8KA8Q@google.com>
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 7bit
Precedence: bulk
MIME-Version: 1.0
X-MS-Exchange-AntiSpam-MessageData-ChunkCount: 1
X-MS-Exchange-AntiSpam-MessageData-0:
	=?utf-8?B?NWltT1dHbkF5REVGSzB5UHhBSzVjYWxEalpmV1pIMG9pckphTGwvQ2hWcXdB?=
 =?utf-8?B?UkgxcG8vYUhua21nc3hDTm5DTlMwN3JDN3JETHF5elFsYVZKUXZnUlJEZ0hi?=
 =?utf-8?B?TmFlajZtdUplK2cxbDIzWGJqSzdQV3NOajgxMkhSRU9LMGZKdFo1SXVHbDhM?=
 =?utf-8?B?Ri9FMzFkNElxWGhERGhpYkVWei83a3BRYkowajl4QnNEVEV6WGl3UW05NjZ6?=
 =?utf-8?B?KzBYSU1MNElPNDVxVWh3aGVqRnNNbFlSOWZuWlFaMkI0UWY1VkthK3FxV3Bm?=
 =?utf-8?B?Q0RVV2J5Vzl1ZzVwaXV1UHVmZEFWSTVlbU1PdklZN3Frbm4rZ2paT1FQYlZX?=
 =?utf-8?B?OTRDRWFHRS9qTVR1MDd0WjFwUmh1Y2NuVEhzRXY5L0VPd1UyUkdDMmxRdHln?=
 =?utf-8?B?dXNkV2g5UWV1cGZyU05WNVVDWlR6aGJoZGZoN0x2cUZ0RVcxNGtKSzQzaUxz?=
 =?utf-8?B?cEZRMjdxdE4zSHVPRzlDc0NMMmZyd29UcDAvWThZVjQzWDFVY3J3NktoTzRF?=
 =?utf-8?B?UjA1bjliT0NMLzNibGc0VHF3dW9kWnpSc2hVV3NjQjNJaTZ4TTZkQmlxMFdV?=
 =?utf-8?B?NzVMdUpjbFV6NFRZQ3NhNzB2clZKOGRTcVhuY0hiUHh3WS9LKzFLOGJlemtE?=
 =?utf-8?B?WVhjTVdkQU5oTXNudWJQV1dZMDlSUVhvK0s2NjFWZkQ0SDBSWjBmMHRnTU5s?=
 =?utf-8?B?RVpITWZZV1MwUzlLTTc4eGRxMjlHUU8wcjBZcGNKTVkwRDgwUEQ4RnpBSlRS?=
 =?utf-8?B?bmliQzhnTERBMlE0ZmY3UVJFTXRhS3dTZFRhazVJc0h2SEdpM1JQUFVGaUZ6?=
 =?utf-8?B?Zk1sbjJ5cTZ6a0phcVdPRldFMS8xL3dRTjhJTGJ4bGM5bWFmQjZXcVludHpK?=
 =?utf-8?B?WnhNVWRZKzhKU3dsU01tVjhGNFVDdHN6S0dmdVJWVWd2QWhjYnBNYm1ET21t?=
 =?utf-8?B?RlRzbjdwMUQzSFlObk1JU0t5RVdLdXdMd1lFWmVsc0dOMWdxWHV2Q2FWaGlS?=
 =?utf-8?B?ZUN4bXZKUHpNTTA5eFpDNHhQcEM2cTZIRDJrQUEwckVGNVdMbFdyUEZhb1k1?=
 =?utf-8?B?MHBFdTBubjRSS1JsWTNMUFJJekxxU2hsdUIzN1N3bldOUTYvR0lwcTNJdWk1?=
 =?utf-8?B?N1h1Y0hmSmFTM01vZjJrRXpTeFIrL3I2ZHFkc0lKNzFSRGlQRm5kMEFGaVBW?=
 =?utf-8?B?a250YVZVTjNTdWRZRHdlQ0dROVpMR0dyV1hYVFZPeWc3L3c5WUplWXhkZE1X?=
 =?utf-8?B?ZHdWd3RGMVpRL2Z2b0FXMlo4NjhLbjBaUWxac0lwbWtHR3dlaUFSQnhhbkxq?=
 =?utf-8?B?WnMxL0xvWGVMUldZakRwcExhUjBOZjRRRWpuUzdLTStyV3lBYXJTTER6MGlL?=
 =?utf-8?B?RldnSVQzUkxHd2JJVjVyMjhOVS96SC9ocFRTcEJHNFFjbUdMaEVreEsrc1l3?=
 =?utf-8?B?aitoS0ZiYzdFOWZyZVA2L0xDWkhIYnRUcjNicGRDU2JvQ1RPWjNUaHdUNzBT?=
 =?utf-8?B?c1RJenpuVkZJUkxLNmliZlpBNGQvVDZxbXU0K2x1VXduQlkwWEhQRnZtaEF3?=
 =?utf-8?B?K2s3RUdNTVkvR0g4dERFQnF1YkhZWUphb25YVEk4K3pEQjliYTdPTmZWV1hO?=
 =?utf-8?B?d3lQUU5CZSs5SkdLOWVWc21vK0Z3eTBQc2FrL2Y1SllFbUFmbUxHTVJ0bjFn?=
 =?utf-8?B?SlhLMzloNDAzb1FabHMxZ014SXZnZkpPa0t6bXMvbXFpR1N0Q2wxbFRIa2NL?=
 =?utf-8?B?bzZiZ2tRQUdCR0hoamhUVXBEQW9xOXQvbG9VYjFBWmI5emxGcURnWWJnZzFT?=
 =?utf-8?B?aE9hV0FoVVF6NEF4Nkh2L3p6Ny95T1U4aHBzMVNOUzBiaFljL2g2Z3BjajNU?=
 =?utf-8?B?YVJ5TytNRGdxb0VYbkNxbTR2OTBhUVR2R3ZhMGxXcTRJdm5Vb2JKY3ZVa3lN?=
 =?utf-8?B?aExXWXNLUnlib21aRkU4d0JjN2tGOUx5SXkvZnVQVzNwYlNEUEx1Z3IzV2ov?=
 =?utf-8?B?em5GbTg2MituVmltdWRXUXRycVNJSFZ2bkl1U3JFbFRIRHVETzZpaU52UFZG?=
 =?utf-8?B?NStXVHJMT3pNR25oeW01ampzOHFxRnZqTEJhZGV2UW5ZNVVYeTJLdlRRbSt2?=
 =?utf-8?Q?xaKTPcFSX3E4KQiNuY8mk62yG?=
X-OriginatorOrg: amd.com
X-MS-Exchange-CrossTenant-Network-Message-Id: 20166cc4-f6e9-444c-a402-08dc857a9661
X-MS-Exchange-CrossTenant-AuthSource: DS0PR12MB6584.namprd12.prod.outlook.com
X-MS-Exchange-CrossTenant-AuthAs: Internal
X-MS-Exchange-CrossTenant-OriginalArrivalTime: 05 Jun 2024 16:14:35.0925
 (UTC)
X-MS-Exchange-CrossTenant-FromEntityHeader: Hosted
X-MS-Exchange-CrossTenant-Id: 3dd8961f-e488-4e60-8e11-a82d994e183d
X-MS-Exchange-CrossTenant-MailboxType: HOSTED
X-MS-Exchange-CrossTenant-UserPrincipalName: jlR+MxTtiBiBHGPxZBnEDxRhOEiWE2Ik67NDZzs1DN4jCUSWZZKCTRs+KIHh6iItalwe4ZRZ9qb/gpE631Zxhw==
X-MS-Exchange-Transport-CrossTenantHeadersStamped: SN7PR12MB6768

On 6/5/2024 8:38 PM, Sean Christopherson wrote:
> On Wed, Jun 05, 2024, Ravi Bangoria wrote:
>> Hi Sean,
>>
>> On 6/4/2024 6:15 AM, Sean Christopherson wrote:
>>> On Mon, Apr 29, 2024, Ravi Bangoria wrote:
>>>> Upcoming AMD uarch will support Bus Lock Detect. Add support for it
>>>> in KVM. Bus Lock Detect is enabled through MSR_IA32_DEBUGCTLMSR and
>>>> MSR_IA32_DEBUGCTLMSR is virtualized only if LBR Virtualization is
>>>> enabled. Add this dependency in the KVM.
>>>
>>> This is woefully incomplete, e.g. db_interception() needs to be updated to decipher
>>> whether the #DB is the responsbility of the host or of the guest.
>>
>> Can you please elaborate. Are you referring to vcpu->guest_debug thingy?
> 
> Yes.  More broadly, all of db_interception().
> 
>>> Honestly, I don't see any point in virtualizing this in KVM.  As Jim alluded to,
>>> what's far, far more interesting for KVM is "Bus Lock Threshold".  Virtualizing
>>> this for the guest would have been nice to have during the initial split-lock #AC
>>> support, but now I'm skeptical the complexity is worth the payoff.
>>
>> This has a valid usecase of penalizing offending processes. I'm not sure
>> how much it's really used in the production though.
> 
> Yeah, but split-lock #AC and #DB have existed on Intel for years, and no one has
> put in the effort to land KVM support, despite the series getting as far as v9[*].

Split-Lock Detect through #AC and Bus Lock Detect through #DB are independent
features. AMD supports only Bus Lock Detect with #DB. I'm not sure about Split
Lock Detect but Intel supports Bus Lock Detect in the guest. These are the
relevant commits:

  https://git.kernel.org/torvalds/c/9a3ecd5e2aa10
  https://git.kernel.org/torvalds/c/e8ea85fb280ec
  https://git.kernel.org/torvalds/c/76ea438b4afcd

> Some of the problems on Intel were due to the awful FMS-based feature detection,
> but those weren't the only hiccups.  E.g. IIRC, we never sorted out what should
> happen if both the host and guest want bus-lock #DBs.

I've to check about vcpu->guest_debug part, but keeping that aside, host and
guest can use Bus Lock Detect in parallel because, DEBUG_CTL MSR and DR6
register are save/restored in VMCB, hardware cause a VMEXIT_EXCEPTION_1 for
guest #DB(when intercepted) and hardware raises #DB on host when it's for the
host.

Please correct me if I misunderstood your comment.

> Anyways, my point is that, except for SEV-ES+ where there's no good reason NOT to
> virtualize Bus Lock Detect, I'm not convinced that it's worth virtualizing bus-lock
> #DBs.
> 
> [*] https://lore.kernel.org/all/20200509110542.8159-1-xiaoyao.li@intel.com
> 
>>> I suppose we could allow it if #DB isn't interecepted, at which point the enabling
>>> required is minimal?
>>
>> The feature uses DEBUG_CTL MSR, #DB and DR6 register. Do you mean expose
>> it when all three are accelerated or just #DB?
> 
> I mean that if KVM isn't intercepting #DB, then there's no extra complexity needed
> to sort out whether the #DB "belongs" to the host or the guest.  See commit
> 90cbf6d914ad ("KVM: SEV-ES: Eliminate #DB intercept when DebugSwap enabled").