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Re: [PATCH v2 0/3] target/i386: allow using named CPU modeles with user
From: |
Daniel P . Berrangé |
Subject: |
Re: [PATCH v2 0/3] target/i386: allow using named CPU modeles with user mode emulation |
Date: |
Mon, 26 Jun 2023 10:20:45 +0100 |
User-agent: |
Mutt/2.2.9 (2022-11-12) |
On Mon, Jun 26, 2023 at 10:33:14AM +0200, Paolo Bonzini wrote:
> When running qemu-i386 even with -cpu Nehalem (aka x86_64-v2), it raises the
> warning
>
> TCG doesn't support requested feature: CPUID.80000001H:EDX.lm [bit 29]
>
> Likewise using qemu-x86_64 with -cpu Haswell, it raises warnings for
>
> TCG doesn't support requested feature: CPUID.01H:ECX.pcid [bit 17]
> TCG doesn't support requested feature: CPUID.01H:ECX.x2apic [bit 21]
> TCG doesn't support requested feature: CPUID.01H:ECX.tsc-deadline [bit 24]
> TCG doesn't support requested feature: CPUID.07H:EBX.invpcid [bit 10]
>
> Even though x86_64-v3 is the level that qemu-user is supposed to provide.
> These are two related problems:
>
> - for Haswell, the 4 missing features are only relevant to code running
> in kernel mode; we can ignore them and mark them as supported even
> though they are not. The supported features are only used to print
> the warnings
>
> - for qemu-i386, the issue is that this emulator is restricted to running
> programs on a 32-bit processor, even though it is actually emulating
> programs that use the 32-bit kernel ABI on any processor. The behavior
> the processor in compatibility (32-bit CS, EFER.LMA=1) mode is mostly
> the same as when EFER.LMA=0 and the little code that is needed to handle
> 64-bit ring-0 is even present in the qemu-i386 binary, just hidden
> behind checks for HF_LMA_MASK. So, LM can also be treated as a feature
> that is only relevant in kernel mode; not entirely, so this cannot yet
> be extended to bsd-user, but the required changes are minimal.
IMHO in this case we should hard block all named CPUs with
'lm' set from qemu-i386. It only makes sense to use named CPU
models that were actually from the 32-bit era with qemu-i386.
If someone wants Nehalem then they should be using qemu-x86_64.
If someone wants qemu-i386 then they should be using an older
named CPU model predating 'lm'.
With regards,
Daniel
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