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Re: [Xen-devel] [PATCH for 4.13] x86/shim: don't reserve 640k - 1M region in E820


  • To: Sergey Dyasli <sergey.dyasli@xxxxxxxxxx>, Jan Beulich <jbeulich@xxxxxxxx>
  • From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
  • Date: Tue, 29 Oct 2019 10:41:51 +0000
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  • Cc: Juergen Gross <jgross@xxxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>, Wei Liu <wl@xxxxxxx>, xen-devel@xxxxxxxxxxxxx
  • Delivery-date: Tue, 29 Oct 2019 10:41:58 +0000
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  • List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
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On 29/10/2019 10:40, Sergey Dyasli wrote:
> On 29/10/2019 10:19, Andrew Cooper wrote:
>> On 29/10/2019 10:17, Sergey Dyasli wrote:
>>> On 28/10/2019 11:33, Andrew Cooper wrote:
>>>> For now, how about cpu_has_hypervisor ?
>>>>
>>>> Whatever the virtual environment, we should trust the provided memory map.
>>> Unfortunately, this plan has failed: init_e820() is called way before
>>> early_cpu_init() and testing cpu_has_hypervisor is meaningless there.
>>>
>>> I guess I'll go for !pvh_boot check for now.
>> Opencode it with cpuid_ecx(1) for now, as we've done elsewhere.
>>
>> I've got a patch in my not-progressing-very-quickly series to remove the
>> boot time mapping at 0 which cleans up early CPUID handling so this can
>> turn into cpu_has_hypervisor, but that is definitely post-4.13 material
>> right now.
> Looking closely at early_cpu_init(), I don't see anything there that
> would prevent placing it near the top of __start_xen(). It works for
> pv-shim, but I need to also do some bare metal testing with this change.
>
> Do you know of anything that would prevent it?

Yes - microcode loading.  That sequence of things in __start_xen() is
very position sensitive.

~Andrew

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