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Thu, 21 Mar 2024 03:18:27 -0700 (PDT) Message-ID: Date: Thu, 21 Mar 2024 11:18:25 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] x86/vcpu: relax VCPUOP_initialise restriction for non-PV vCPUs Content-Language: en-US To: =?UTF-8?Q?Roger_Pau_Monn=C3=A9?= Cc: Andrew Cooper , George Dunlap , Julien Grall , Stefano Stabellini , Wei Liu , xen-devel@lists.xenproject.org References: <20240320135720.68894-1-roger.pau@citrix.com> <73314dc5-0145-4aa7-a3cd-23c943235392@suse.com> <4cc5d746-a50c-4819-a56e-512abf797ef2@suse.com> From: Jan Beulich Autocrypt: addr=jbeulich@suse.com; keydata= xsDiBFk3nEQRBADAEaSw6zC/EJkiwGPXbWtPxl2xCdSoeepS07jW8UgcHNurfHvUzogEq5xk hu507c3BarVjyWCJOylMNR98Yd8VqD9UfmX0Hb8/BrA+Hl6/DB/eqGptrf4BSRwcZQM32aZK 7Pj2XbGWIUrZrd70x1eAP9QE3P79Y2oLrsCgbZJfEwCgvz9JjGmQqQkRiTVzlZVCJYcyGGsD /0tbFCzD2h20ahe8rC1gbb3K3qk+LpBtvjBu1RY9drYk0NymiGbJWZgab6t1jM7sk2vuf0Py O9Hf9XBmK0uE9IgMaiCpc32XV9oASz6UJebwkX+zF2jG5I1BfnO9g7KlotcA/v5ClMjgo6Gl MDY4HxoSRu3i1cqqSDtVlt+AOVBJBACrZcnHAUSuCXBPy0jOlBhxPqRWv6ND4c9PH1xjQ3NP nxJuMBS8rnNg22uyfAgmBKNLpLgAGVRMZGaGoJObGf72s6TeIqKJo/LtggAS9qAUiuKVnygo 3wjfkS9A3DRO+SpU7JqWdsveeIQyeyEJ/8PTowmSQLakF+3fote9ybzd880fSmFuIEJldWxp Y2ggPGpiZXVsaWNoQHN1c2UuY29tPsJgBBMRAgAgBQJZN5xEAhsDBgsJCAcDAgQVAggDBBYC AwECHgECF4AACgkQoDSui/t3IH4J+wCfQ5jHdEjCRHj23O/5ttg9r9OIruwAn3103WUITZee e7Sbg12UgcQ5lv7SzsFNBFk3nEQQCACCuTjCjFOUdi5Nm244F+78kLghRcin/awv+IrTcIWF hUpSs1Y91iQQ7KItirz5uwCPlwejSJDQJLIS+QtJHaXDXeV6NI0Uef1hP20+y8qydDiVkv6l IreXjTb7DvksRgJNvCkWtYnlS3mYvQ9NzS9PhyALWbXnH6sIJd2O9lKS1Mrfq+y0IXCP10eS FFGg+Av3IQeFatkJAyju0PPthyTqxSI4lZYuJVPknzgaeuJv/2NccrPvmeDg6Coe7ZIeQ8Yj t0ARxu2xytAkkLCel1Lz1WLmwLstV30g80nkgZf/wr+/BXJW/oIvRlonUkxv+IbBM3dX2OV8 AmRv1ySWPTP7AAMFB/9PQK/VtlNUJvg8GXj9ootzrteGfVZVVT4XBJkfwBcpC/XcPzldjv+3 HYudvpdNK3lLujXeA5fLOH+Z/G9WBc5pFVSMocI71I8bT8lIAzreg0WvkWg5V2WZsUMlnDL9 mpwIGFhlbM3gfDMs7MPMu8YQRFVdUvtSpaAs8OFfGQ0ia3LGZcjA6Ik2+xcqscEJzNH+qh8V m5jjp28yZgaqTaRbg3M/+MTbMpicpZuqF4rnB0AQD12/3BNWDR6bmh+EkYSMcEIpQmBM51qM EKYTQGybRCjpnKHGOxG0rfFY1085mBDZCH5Kx0cl0HVJuQKC+dV2ZY5AqjcKwAxpE75MLFkr wkkEGBECAAkFAlk3nEQCGwwACgkQoDSui/t3IH7nnwCfcJWUDUFKdCsBH/E5d+0ZnMQi+G0A nAuWpQkjM1ASeQwSHEeAWPgskBQL In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit On 21.03.2024 10:57, Roger Pau Monné wrote: > On Thu, Mar 21, 2024 at 10:17:25AM +0100, Jan Beulich wrote: >> On 21.03.2024 10:10, Roger Pau Monné wrote: >>> On Thu, Mar 21, 2024 at 09:07:10AM +0100, Jan Beulich wrote: >>>> On 20.03.2024 17:29, Roger Pau Monné wrote: >>>>> On Wed, Mar 20, 2024 at 04:09:33PM +0100, Jan Beulich wrote: >>>>>> On 20.03.2024 14:57, Roger Pau Monne wrote: >>>>>>> There's no reason to force HVM guests to have a valid vcpu_info area when >>>>>>> initializing a vCPU, as the vCPU can also be brought online using the local >>>>>>> APIC, and on that path there's no requirement for vcpu_info to be setup ahead >>>>>>> of the bring up. Note an HVM vCPU can operate normally without making use of >>>>>>> vcpu_info. >>>>>> >>>>>> While I'd agree if you started with "There's no real need to force ...", I >>>>>> still think there is a reason: If one wants to use paravirt interfaces (i.e. >>>>>> hypercalls), they would better do so consistently. After all there's also >>>>>> no need to use VCPUOP_initialise, yet you're not disabling its use. >>>>>> >>>>>> As said in reply to Andrew's reply, besides acting as a sentinel that >>>>>> structure instance also acts as a sink for Xen accesses to a vCPU's >>>>>> vcpu_info. By removing the check, you switch that from being a safeguard to >>>>>> being something that actually has to be expected to be accessed. Unless >>>>>> you've audited all uses to prove that no such access exists. >>>>> >>>>> I'm kind of lost in this last paragraph, how is that different than >>>>> what currently happens when an HVM vCPU >= 32 is brought up using the >>>>> lapic and has no vpcu_info mapped? >>>> >>>> I think this aspect was simply missed back at the time. And I think it >>>> wants mentioning explicitly to justify the change. >>> >>> OK, I can add to the commit message: >>> >>> "Note an HVM vCPU can operate normally without making use of >>> vcpu_info, and in fact does so when brought up from the local APIC." >> >> I'd be fine adding this (or having this added) while committing. >> >>>> As said in reply to Andrew, I don't think the dummy structure can be got >>>> rid of. Nor can the checks here be (easily) removed altogether, i.e. your >>>> change cannot (easily) be extended to PV as well. Even conditional removal >>>> of the structure in !PV builds would first require all vcpu_info accesses >>>> to gain a suitable conditional. Which may be undesirable, as some of these >>>> may be deemed fast paths. >>> >>> I didn't intended to do this here, as replied to Andrew. If we want >>> to get rid of the check for PV also it needs to be done in a different >>> patch, and with a different justification and analysis. >>> >>>>> Also, from a quick look it seems like sites do check whether vcpu_info >>>>> == dummy_vcpu_info, otherwise we would already be in trouble. >>>> >>>> Such checks exist in code managing vcpu_info, but not - afaics - in places >>>> actually accessing it. > > Is there anything else that needs adjusting then, or are you happy to > pick this up and adjust the commit message? I'll pick this up and adjust (unless Andrew beats me). Jan