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Wed, 27 Mar 2024 00:15:30 -0700 (PDT) Message-ID: Date: Wed, 27 Mar 2024 08:15:29 +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: Julien Grall Cc: xen-devel@lists.xenproject.org, =?UTF-8?Q?Roger_Pau_Monn=C3=A9?= , George Dunlap , Stefano Stabellini , Wei Liu , Andrew Cooper References: <20240320135720.68894-1-roger.pau@citrix.com> <1304a0d2-ff17-4823-9d31-c32eb7b79f30@citrix.com> <815d5bd4-18af-4f5f-b6a5-6302a1d682a2@citrix.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 26.03.2024 23:08, Julien Grall wrote: > Hi Andrew, > > On 20/03/2024 14:39, Andrew Cooper wrote: >> On 20/03/2024 2:26 pm, Roger Pau Monné wrote: >>> On Wed, Mar 20, 2024 at 02:06:27PM +0000, Andrew Cooper wrote: >>>> On 20/03/2024 1:57 pm, 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. >>>>> >>>>> Restrict the check against dummy_vcpu_info to only apply to PV guests. >>>>> >>>>> Fixes: 192df6f9122d ('x86: allow HVM guests to use hypercalls to bring up vCPUs') >>>>> Signed-off-by: Roger Pau Monné >>>> Thanks for looking into this.  But, do we actually need to force this on >>>> PV either? >>> Possibly, having now taken a look at the users it does seem they could >>> cope with unpopulated vcpu_info_area. >>> >>> Part of my understanding was that this was some kind of courtesy to PV >>> guests in order to prevent starting them without a vcpu_info, which >>> strictly speaking is not mandatory, but otherwise the guest vCPU won't >>> be able to receive interrupts, not even IPIs. >> >> That's more of a stick than a carrot... "you must set up the area of a >> CPU before you can bring it online". Except as we've seen, HVM has been >> fine all along without it. >>>> The only interesting user of dummy_vcpu_info now is vcpu_info_populate() >>>> which can cope with any arbitrary vCPU. >>>> >>>> I have a suspicion that we can (now) delete these two checks, delete the >>>> dummy_vcpu_info object, and use a regular NULL pointer in >>>> vcpu_info_{populate,reset}(), and in doing so, remove one of the bigger >>>> pieces of PV-absurdity from Xen. >>> I was expecting this to be something we can backport. OTOH removing >>> the check completely (or even getting rid of dummy_vcpu_info) is not >>> something that we would want to backport. >>> >>> I would rather do the removal of dummy_vcpu_info as followup work. >> >> I was worried about ARM with your patch, because it's spelt HVM there, >> rather than PV. >> >> However, I'd forgotten that ARM's do_vcpu_op() filters ops down to just >> VCPUOP_register_{vcpu_info,runstate_memory_area} so VCPUOP_initialise >> isn't reachable. >> >> Therefore, Reviewed-by: Andrew Cooper >> >> It also means ARM can't use any of the PHYS registration yet... > > Whoops. I don't think this was intended. Jan, I don't seem to find any > use in Linux. Do you have any patches you could share? No, I don't. I did all development with hacked up XTF tests, and I was expecting Linux folks to be looking into making use of the new subops. Jan > I would like to > give a try on Arm before sending a patch? > > Cheers, >