Re: [PATCH v2 3/5] KVM: SVM: Manually context switch DEBUGCTL if LBR virtualization is disabled

[Sean Christopherson <seanjc@google.com>]

On Thu, Feb 27, 2025, Ravi Bangoria wrote:
> > Somewhat related but independent: CPU automatically clears DEBUGCTL[BTF]
> > on #DB exception. So, when DEBUGCTL is save/restored by KVM (i.e. when
> > LBR virtualization is disabled), it's KVM's responsibility to clear
> > DEBUGCTL[BTF].
> 
> Found this with below KUT test.
> 
> (I wasn't sure whether I should send a separate series for kernel fix + KUT
> patch, or you can squash kernel fix in your patch and I shall send only KUT
> patch. So for now, sending it as a reply here.)

Actualy, I'll post this along with some other cleanups to the test, and a fix
for Intel if needed (it _should_ pass on Intel).  All the open-coded EFLAGS.TF
literals can be replaced, and clobbering arithmetic flags with SS is really, really,
gross.

> ---
> diff --git a/x86/debug.c b/x86/debug.c
> index f493567c..2d204c63 100644
> --- a/x86/debug.c
> +++ b/x86/debug.c
> @@ -409,6 +409,45 @@ static noinline unsigned long singlestep_with_sti_hlt(void)
>  	return start_rip;
>  }
>  
> +static noinline unsigned long __run_basic_block_ss_test(void)
> +{
> +	unsigned long start_rip;
> +
> +	wrmsr(MSR_IA32_DEBUGCTLMSR, DEBUGCTLMSR_BTF);
> +
> +	asm volatile(
> +		"pushf\n\t"
> +		"pop %%rax\n\t"
> +		"or $(1<<8),%%rax\n\t"
> +		"push %%rax\n\t"
> +		"popf\n\t"
> +		"1: nop\n\t"
> +		"jmp 2f\n\t"
> +		"nop\n\t"
> +		"2: lea 1b(%%rip), %0\n\t"
> +		: "=r" (start_rip) : : "rax"
> +	);
> +
> +	return start_rip;
> +}
> +
> +static void run_basic_block_ss_test(void)
> +{
> +	unsigned long jmp_target;
> +	unsigned long debugctl;
> +
> +	write_dr6(0);
> +	jmp_target = __run_basic_block_ss_test() + 4;
> +
> +	report(is_single_step_db(dr6[0]) && db_addr[0] == jmp_target,
> +	       "Basic Block Single-step #DB: 0x%lx == 0x%lx", db_addr[0],
> +	       jmp_target);
> +
> +	debugctl = rdmsr(MSR_IA32_DEBUGCTLMSR);
> +	/* CPU should automatically clear DEBUGCTL[BTF] on #DB exception */
> +	report(debugctl == 0, "DebugCtl[BTF] reset post #DB. 0x%lx", debugctl);
> +}
> +
>  int main(int ac, char **av)
>  {
>  	unsigned long cr4;
> @@ -475,6 +514,12 @@ int main(int ac, char **av)
>  	run_ss_db_test(singlestep_with_movss_blocking_and_dr7_gd);
>  	run_ss_db_test(singlestep_with_sti_hlt);
>  
> +	/* Seems DEBUGCTL[BTF] is not supported on Intel. Run it only on AMD */
> +	if (this_cpu_has(X86_FEATURE_SVM)) {
> +		n = 0;
> +		run_basic_block_ss_test();
> +	}
> +
>  	n = 0;
>  	write_dr1((void *)&value);
>  	write_dr6(DR6_BS);
> ---
> 
> Thanks,
> Ravi