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Subject: Re: [PATCH RFC 2/3] x86/xen/time: setup vcpu 0 time info page
To: Joao Martins <joao.m.martins@oracle.com>, linux-kernel@vger.kernel.org,
        xen-devel@lists.xen.org
References: <1451339557-24473-1-git-send-email-joao.m.martins@oracle.com>
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 <568A98D9.9090501@oracle.com> <568AD8F3.2050600@oracle.com>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
        David Vrabel <david.vrabel@citrix.com>,
        Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>,
        "H. Peter Anvin" <hpa@zytor.com>, x86@kernel.org
From: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Message-ID: <568AE553.5030303@oracle.com>
Date: Mon, 4 Jan 2016 16:34:11 -0500
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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(..);

-boris