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Re: [Xen-devel] [PATCH v2 2/4] x86/microcode: avoid unnecessary xmalloc/memcpy of ucode data
- To: Jan Beulich <jbeulich@xxxxxxxx>, Eslam Elnikety <elnikety@xxxxxxxxxx>
- From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- Date: Fri, 17 Jan 2020 21:05:52 +0000
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- Cc: Stefano Stabellini <sstabellini@xxxxxxxxxx>, Julien Grall <julien@xxxxxxx>, Wei Liu <wl@xxxxxxx>, Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>, George Dunlap <George.Dunlap@xxxxxxxxxxxxx>, Ian Jackson <ian.jackson@xxxxxxxxxxxxx>, Paul Durrant <pdurrant@xxxxxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxxx, David Woodhouse <dwmw@xxxxxxxxxxxx>
- Delivery-date: Fri, 17 Jan 2020 21:06:18 +0000
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- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
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On 20/12/2019 09:57, Jan Beulich wrote:
> On 19.12.2019 22:25, Eslam Elnikety wrote:
>> On 18.12.19 13:05, Jan Beulich wrote:
>>> On 18.12.2019 02:32, Eslam Elnikety wrote:
>>>> @@ -725,7 +701,7 @@ static int __init microcode_init(void)
>>>> */
>>>> if ( ucode_blob.size )
>>>> {
>>>> - xfree(ucode_blob.data);
>>>> + bootstrap_map(NULL);
>>> As much as I like the change, I wholeheartedly disagree with this
>>> aspect of it: You make it largely unpredictable when the boot
>>> mappings will go away - it becomes entirely dependent upon link
>>> order. And of course we really want these mappings to be gone,
>>> the very latest (I think), by the time we start bringing up APs
>>> (but generally the sooner the better). This is (one of?) the main
>>> reason(s) why it hadn't been done this way to begin with. The
>>> alternative is more complicated (set up a proper, long term
>>> mapping), but it's going to be more clean (including the mapping
>>> then also being suitable to post-boot CPU onlining).
>>>
>> This change is an improvement on the current status. We get to avoid
>> xmalloc/memcpy in the case of a successful ucode=scan. The problematic
>> aspect you highlight is anyway there regardless of this patch (ref. to
>> the "else if ( ucode_mod.mod_end )" in microcode_init).
> Hmm, fair point. I'm not a fan of making a bad situation worse,
> but I think it's acceptable here:
> Acked-by: Jan Beulich <jbeulich@xxxxxxxx>
Specifically relevant to this conversation is point 2 of
https://lore.kernel.org/xen-devel/20200109193241.14542-1-andrew.cooper3@xxxxxxxxxx/
where having dynamic bootmap mappings seems pointless when all we're
doing is mapping RAM below 4G.
~Andrew
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