Re: [Qemu-devel] [PATCH] Revert "i386: correct cpu_x86_cpuid(0xd)"

[address@hidden]

It is CentOS 6.3 with kernel version 2.6.32-279. Actually all CentOS 6 releases 
have this issue.



address@hidden
 
From: Eduardo Habkost
Date: 2019-08-21 19:19
To: address@hidden
CC: qemu-devel
Subject: Re: Re: [Qemu-devel] [PATCH] Revert "i386: correct cpu_x86_cpuid(0xd)"
On Wed, Aug 21, 2019 at 11:04:46AM +0800, address@hidden wrote:
> Thanks for you reply, we have some hosts running with legacy kernel, 
> difficult to upgrade, and i want to run the latest qemu.
> Does QEMU support running with legacy kernel(kvm) in design?
 
For KVM, QEMU requires Linux 4.5 or newer.  See "System
requirements" / "KVM kernel module" section on qemu-doc.  We also
aim to support the latest version of Linux distributions with
long term support (e.g. RHEL, Debian, Ubuntu LTS, SLES).
 
Do you have more details on the kernel you are using?  Is it
built and distributed by a third party?
 
 
> 
> 
> 
> address@hidden
>  
> From: Eduardo Habkost
> Date: 2019-08-21 05:23
> To: Bingsong Si
> CC: qemu-devel
> Subject: Re: [Qemu-devel] [PATCH] Revert "i386: correct cpu_x86_cpuid(0xd)"
> On Mon, Aug 19, 2019 at 06:09:24PM +0800, Bingsong Si wrote:
> > This reverts commit de2e68c902f7b6e438b0fa3cfedd74a06a20704f.
> > 
> > Initial value of env->xcr0 == 0, then CPUID(EAX=0xd,ECX=0).EBX == 0, after 
> > kvm
> > upstream commit 412a3c41, It is ok.
> > On host before commit 412a3c41, some legacy guest, i.e. CentOS 6, get
> > xstate_size == 0, will crash the guest.
> > 
> > Signed-off-by: Bingsong Si <address@hidden>
>  
> cpu_x86_cpuid() is also used by TCG, and needs to return the
> correct data depending on xcr0.  If you want to work around a KVM
> bug by ignoring xcr0, it needs to be conditional on
> kvm_enabled().
>  
> But even if we you make this conditional on kvm_enabled(), I
> don't understand why QEMU would need a workaround for a KVM bug
> that was fixed more than 4 years ago.
>  
> > ---
> >  target/i386/cpu.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/target/i386/cpu.c b/target/i386/cpu.c
> > index ff65e11008..69562e21ed 100644
> > --- a/target/i386/cpu.c
> > +++ b/target/i386/cpu.c
> > @@ -4416,7 +4416,7 @@ void cpu_x86_cpuid(CPUX86State *env, uint32_t index, 
> > uint32_t count,
> >              *ecx = xsave_area_size(x86_cpu_xsave_components(cpu));
> >              *eax = env->features[FEAT_XSAVE_COMP_LO];
> >              *edx = env->features[FEAT_XSAVE_COMP_HI];
> > -            *ebx = xsave_area_size(env->xcr0);
> > +            *ebx = *ecx;
> >          } else if (count == 1) {
> >              *eax = env->features[FEAT_XSAVE];
> >          } else if (count < ARRAY_SIZE(x86_ext_save_areas)) {
> > -- 
> > 2.22.0
> > 
> > 
>  
> -- 
> Eduardo
 
-- 
Eduardo