Re: qemu-user self emulation broken with default CPU on x86/x64

["Daniel P. Berrangé" <berrange@redhat.com>]

On Mon, Jul 03, 2023 at 06:03:08PM +0200, Pierrick Bouvier wrote:
> Hi everyone,
> 
> Recently (in d135f781 [1], between v7.0.0 and v8.0.0), qemu-user default cpu
> was updated to "max" instead of qemu32/qemu64.
> 
> This change "broke" qemu self emulation if this new default cpu is used.
> 
> $ ./qemu-x86_64 ./qemu-x86_64 --version
> qemu-x86_64: ../util/cacheflush.c:212: init_cache_info: Assertion `(isize &
> (isize - 1)) == 0' failed.
> qemu: uncaught target signal 6 (Aborted) - core dumped
> Aborted
> 
> By setting cpu back to qemu64, it works again.
> $ ./qemu-x86_64 -cpu qemu64 ./qemu-x86_64  --version
> qemu-x86_64 version 8.0.50 (v8.0.0-2317-ge125b08ed6)
> Copyright (c) 2003-2023 Fabrice Bellard and the QEMU Project developers
> 
> Commenting assert does not work, as qemu aligned malloc fail shortly after.
> 
> I'm willing to fix it, but I'm not sure what is the issue with "max" cpu
> exactly. Is it missing CPU cache line, or something else?
> Any guidance would be welcome.

Yes, it appears that dcache info being reported by the 'max' CPU is
bogus. We can simply the test case with 'getconf'

With 'max' CPU model:

# getconf -a | grep DCACHE
LEVEL1_DCACHE_SIZE                 7273809
LEVEL1_DCACHE_ASSOC                1
LEVEL1_DCACHE_LINESIZE             2697

with 'qemu64' CPU model

# getconf -a | grep DCACHE
LEVEL1_DCACHE_SIZE                 0
LEVEL1_DCACHE_ASSOC                0
LEVEL1_DCACHE_LINESIZE             0

OR with Nehalem:

# getconf -a | grep DCACHE
LEVEL1_DCACHE_SIZE                 32768
LEVEL1_DCACHE_ASSOC                8
LEVEL1_DCACHE_LINESIZE             64

> 
> I know it's not the most important problem on earth, but it's still
> surprising to meet this when you try to use qemu to emulate itself.
> 
> Regards,
> Pierrick
> 
> [1] https://gitlab.com/qemu-project/qemu/-/commit/d135f781405f7c78153aa65e0327b05a4aa72e50
> 

With regards,
Daniel
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