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Re: [PATCH] x86/shim: Simplify compat handling in write_start_info()



On 19.04.2021 17:57, Andrew Cooper wrote:
> On 19/04/2021 16:55, Jan Beulich wrote:
>> On 19.04.2021 16:45, Andrew Cooper wrote:
>>> Factor out a compat boolean to remove the lfence overhead from multiple
>>> is_pv_32bit_domain() calls.
>>>
>>> For a compat guest, the upper 32 bits of rdx are zero, so there is no need 
>>> to
>>> have any conditional logic at all when mapping the start info page.
>> Iirc the contents of the upper halves hold unspecified contents after
>> a switch from compat to 64-bit mode. Therefore only with this part of
>> the change dropped ...
> 
> But we're shim, so will never ever mix compat and non-compat guests.

That's not the point: A compat guest will still cause the CPU to
transition back and forth between 64-bit and compat modes. It is
this transitioning which leaves the upper halves of all GPRs in
undefined state (even if in reality a CPU would likely need to go
through extra hoops to prevent them from being zero if they were
written to in compat mode).

Jan



 


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