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On versioned CPU models, aliases, and machine types

From: Kashyap Chamarthy
Subject: On versioned CPU models, aliases, and machine types
Date: Fri, 15 Oct 2021 12:52:50 +0200


With the versioned CPU models, I see that the plain, non-versioned named
model now says: "alias configured by machine type".  For example:

    $> qemu-system-x86_64 -cpu help | grep Broadwell
    x86 Broadwell             (alias configured by machine type)                
    x86 Broadwell-IBRS        (alias of Broadwell-v3)                           
    x86 Broadwell-noTSX       (alias of Broadwell-v2)                           
    x86 Broadwell-noTSX-IBRS  (alias of Broadwell-v4)                           
    x86 Broadwell-v1          Intel Core Processor (Broadwell)                  
    x86 Broadwell-v2          Intel Core Processor (Broadwell, no TSX)          
    x86 Broadwell-v3          Intel Core Processor (Broadwell, IBRS)            
    x86 Broadwell-v4          Intel Core Processor (Broadwell, no TSX, IBRS)    

Now, any QEMU binary (qemu-system-x86-6.0.0-1 in my case) supports a ton
of versioned machine types.  So when the plain "Broadwell" model says
"alias configured by machine type", I have two questions:

(1) Is there a way to figure out _which_ exact versioned CPU model will
    plain "Broadwell" alias to in the above case?  

    My guess: Assuming I'm using the plain 'q35' machine type (which
    aliases to the latest versioned machine type), "Broadwell" will
    alias to "Broadwell-v4".

(2) Also, it sounds like depending on the versioned machine type I
    configure, the CPU model alias of plain "Broadwell" might change --
    is this correct?

(Ideally, I'd like to document an example of each of the above in the
in-tree doc, cpu-models-x86.rst.inc.  It's not really obvious.)


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