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

[Eduardo Habkost <ehabkost@redhat.com>]

On Wed, Aug 21, 2019 at 07:54:17PM +0800, owen.si@ucloud.cn wrote:
> It is CentOS 6.3 with kernel version 2.6.32-279. Actually all CentOS 6 releases have this issue.

We stopped supporting CentOS 6 in July 2016 (2 years after CentOS
7 was released).  Be aware that even if we work around that
specific bug, there are no guarantees that QEMU will still build
on a CentOS 6 host in the future.

That said, I probably wouldn't reject a patch that works around
that CentOS 6 bug, if it's conditional on kvm_enabled() and has a
comment explaining why the workaround exists.

> 
> 
> 
> owen.si@ucloud.cn
>  
> From: Eduardo Habkost
> Date: 2019-08-21 19:19
> To: owen.si@ucloud.cn
> 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, owen.si@ucloud.cn 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?
>  
>  
> > 
> > 
> > 
> > owen.si@ucloud.cn
> >  
> > 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 <owen.si@ucloud.cn>
> >  
> > 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

-- 
Eduardo