On 1/29/2026 7:09 AM, Zide Chen wrote:
> BTS (Branch Trace Store), enumerated by IA32_MISC_ENABLE.BTS_UNAVAILABLE
> (bit 11), is deprecated and has been superseded by LBR and Intel PT.
>
> KVM yields control of the above mentioned bit to userspace since KVM
> commit 9fc222967a39 ("KVM: x86: Give host userspace full control of
> MSR_IA32_MISC_ENABLES").
>
> 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.
>
> Signed-off-by: Zide Chen <[email protected]>
> ---
> V2:
> - Address Dapeng's comments.
> - Remove mention of VMState version_id from the commit message.
>
> target/i386/cpu.h | 5 ++++-
> 1 file changed, 4 insertions(+), 1 deletion(-)
>
> diff --git a/target/i386/cpu.h b/target/i386/cpu.h
> index 2bbc977d9088..f02812bfd19f 100644
> --- a/target/i386/cpu.h
> +++ b/target/i386/cpu.h
> @@ -474,8 +474,11 @@ typedef enum X86Seg {
>
> #define MSR_IA32_MISC_ENABLE 0x1a0
> /* Indicates good rep/movs microcode on some processors: */
> -#define MSR_IA32_MISC_ENABLE_DEFAULT 1
> +#define MSR_IA32_MISC_ENABLE_FASTSTRING (1ULL << 0)
> +#define MSR_IA32_MISC_ENABLE_BTS_UNAVAIL (1ULL << 11)
> #define MSR_IA32_MISC_ENABLE_MWAIT (1ULL << 18)
> +#define MSR_IA32_MISC_ENABLE_DEFAULT (MSR_IA32_MISC_ENABLE_FASTSTRING
> |\
> + MSR_IA32_MISC_ENABLE_BTS_UNAVAIL)
>
> #define MSR_MTRRphysBase(reg) (0x200 + 2 * (reg))
> #define MSR_MTRRphysMask(reg) (0x200 + 2 * (reg) + 1)
Reviewed-by: Dapeng Mi <[email protected]>
