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Date:   Mon, 27 Nov 2023 16:00:17 +0100
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Subject: Re: [PATCH] x86/xen: fix percpu vcpu_info allocation
Content-Language: en-US
To:     Boris Ostrovsky <boris.ostrovsky@oracle.com>, x86@kernel.org
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        "H. Peter Anvin" <hpa@zytor.com>, xen-devel@lists.xenproject.org,
        Juergen Gross <jgross@suse.com>, linux-kernel@vger.kernel.org
References: <20231124074852.25161-1-jgross@suse.com>
 <69ba2a99-90f7-4d39-afc6-971b0867a1c8@oracle.com>
From:   Jan Beulich <jbeulich@suse.com>
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On 27.11.2023 15:57, Boris Ostrovsky wrote:
> 
> 
> On 11/24/23 2:48 AM, Juergen Gross wrote:
>> Today the percpu struct vcpu_info is allocated via DEFINE_PER_CPU(),
>> meaning that it could cross a page boundary. In this case registering
>> it with the hypervisor will fail, resulting in a panic().
>>
>> This can easily be fixed by using DEFINE_PER_CPU_ALIGNED() instead,
>> as struct vcpu_info is guaranteed to have a size of 64 bytes, matching
>> the cache line size of x86 64-bit processors (Xen doesn't support
>> 32-bit processors).
>>
>> Fixes: 5ead97c84fa7 ("xen: Core Xen implementation")
>> Signed-off-by: Juergen Gross <jgross@suse.com>
> 
> Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.con>
> 
> although I am not sure in usefulness of BUILD_BUG_ON --- 64 bytes is part of ABI and hypervisor already has its own BUILD_BUG_ON for this.

I understood the check to guard against SMP_CACHE_BYTES < 64.

Jan

>> @@ -160,6 +163,7 @@ void xen_vcpu_setup(int cpu)
>>   	int err;
>>   	struct vcpu_info *vcpup;
>>   
>> +	BUILD_BUG_ON(sizeof(*vcpup) > SMP_CACHE_BYTES);
>>   	BUG_ON(HYPERVISOR_shared_info == &xen_dummy_shared_info);
>