linux-kernel - Re: [PATCH RFC 2/3] x86/xen/time: setup vcpu 0 time info page

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Date:	Tue, 5 Jan 2016 12:20:15 +0000
From:	Joao Martins <joao.m.martins@...cle.com>
To:	Boris Ostrovsky <boris.ostrovsky@...cle.com>
Cc:	linux-kernel@...r.kernel.org, xen-devel@...ts.xen.org,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	David Vrabel <david.vrabel@...rix.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, x86@...nel.org
Subject: Re: [PATCH RFC 2/3] x86/xen/time: setup vcpu 0 time info page



On 01/04/2016 09:34 PM, Boris Ostrovsky wrote:
> On 01/04/2016 03:41 PM, Joao Martins wrote:
>>
>> On 01/04/2016 04:07 PM, Boris Ostrovsky wrote:
>>> On 12/28/2015 04:52 PM, Joao Martins wrote:
>>>
>>>> +
>>>> +	size = PAGE_ALIGN(sizeof(struct pvclock_vsyscall_time_info));
>>>> +	mem = memblock_alloc(size, PAGE_SIZE);
>>>> +	if (!mem)
>>>> +		return -ENOMEM;
>>>> +
>>>> +	ti = __va(mem);
>>>> +	memset(ti, 0, size);
>>> Can you just use get_zeroed_page()? (struct pvclock_vsyscall_time_info
>>> is always less than a page, isn't it?).
>> Yeah, I can use get_zeroed_page() (struct pvclock_vsyscall_time_info is always
>> less than a page).
>>
>> Additionally perhaps this region shouldn't be freed if PVCLOCK_TSC_STABLE_BIT
>> isn't supported, because otherwise I would end up corrupting data elsewhere
>> since the pvti would still be periodically updated by Xen, right?
> 
> You could try setting it back to NULL. e.g.
>      if (!HYPERVISOR_vcpu_op(VCPUOP_register_vcpu_time_memory_area, cpu, 
> NULL))
>          free_page(..);
> 
Ah, forgot about that option. Thanks! We can't register it explicitly as NULL as
the hypercall would return -EFAULT but having a valid argument with NULL as the
address works nicely.

> -boris
> 
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