Re: [PATCH V3 01/13] target/i386: Disable unsupported BTS for guest

[Zhao Liu <zhao1.liu@intel.com>]

On Wed, Mar 04, 2026 at 10:07:00AM -0800, Zide Chen wrote:
> Date: Wed, 4 Mar 2026 10:07:00 -0800
> From: Zide Chen <zide.chen@intel.com>
> Subject: [PATCH V3 01/13] target/i386: Disable unsupported BTS for guest
> X-Mailer: git-send-email 2.53.0
> 
> BTS (Branch Trace Store), enumerated by IA32_MISC_ENABLE.BTS_UNAVAILABLE
> (bit 11), is deprecated and has been superseded by LBR and Intel PT.

I'm not clear from which platform this bit will be set by default?

> KVM yields control of this bit to userspace since KVM commit
> 9fc222967a39 ("KVM: x86: Give host userspace full control of
> MSR_IA32_MISC_ENABLES").

If KVM won't support it, it's better to only configure for KVM.

> However, QEMU does not set this bit, which allows guests to write the
> BTS and BTINT bits in IA32_DEBUGCTL.  Since KVM doesn't support BTS,
> this may lead to unexpected MSR access errors.

But overall, this way is a bit user-unfriendly. For cases where CPUID
is unavailable, it would be more proper to check the KVM API to
determine whether support is available; making this change in userspace
feels a bit like applying the special patch for a corner case.

I found there's another patch where Paolo and Sean didn't want to make
such changes directly earlier on....
https://lore.kernel.org/qemu-devel/20220718032206.34488-1-zhenzhong.duan@intel.com/

Thanks,
Zhao