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Re: [Bug 1904317] [NEW] cpu feature selection is not affected to guest '

From: Philippe Mathieu-Daudé
Subject: Re: [Bug 1904317] [NEW] cpu feature selection is not affected to guest 's cpuid with whpx
Date: Mon, 16 Nov 2020 12:53:46 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.4.0

Cc'ing Sunil (WHPX maintainer).

On 11/15/20 10:06 AM, Takumi Nakamura wrote:
> Public bug reported:
> On windows with -accel whpx, "-cpu" is ignored without any messages.
> Guest recognizes features as same as host's.
> Confirmed on v5.2.0-rc1.
> I suggest qemu may do,
> - Warn with incompatible -cpu options were given.
> - Enhance cpuid handling.
> Background:
> I was investigated mmio and block copy issue in Linux kernel.
> I met a problem that Linux went ill for touching mmio with whpx. (not with 
> tcg)
> I suspect erms(enhanced rep movs) might trigger.
> I tried to mask erms on qemu with -feature,erms, but it was ineffective.
> At last, I disabled erms manually, to tweak whpx-all.c to mask erms in
> cpuid.
> FYI, qemu with whpx from/to mmio, "rep movsb" does byte access regardless of 
> erms.
> Linux kernel tends to choose not "rep movsq" but "rep movsb" with erms.
> ** Affects: qemu
>      Importance: Undecided
>          Status: New

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