Re: [Xen-devel] [PATCH 1/6] x86/vmx: Fix handing of MSR_DEBUGCTL on VMExit

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 28.05.18 at 16:27, <andrew.cooper3@xxxxxxxxxx> wrote:
> Currently, whenever the guest writes a nonzero value to MSR_DEBUGCTL, Xen
> updates a host MSR load list entry with the current hardware value of
> MSR_DEBUGCTL.  This is wrong.

"This is wrong" goes too far for my taste: It is not very efficient to do it 
that
way, but it's still correct. Unless, of course, the zeroing of the register
happens after the processing of the MSR load list (which I doubt it does).

> Initially, I tried to have a common xen_msr_debugctl variable, but
> rip-relative addresses don't resolve correctly in alternative blocks.
> LBR-only has been fine for ages, and I don't see that changing any time 
> soon.

The chosen solution is certainly fine, but the issue could have been
avoided by doing the load from memory ahead of the alternative block
(accepting that it also happens when the value isn't actually needed).

Another option would be to invert the sense of the feature flag,
patching NOPs over the register setup plus WRMSR.

> @@ -1764,17 +1765,6 @@ void do_device_not_available(struct cpu_user_regs 
> *regs)
>      return;
>  }
>  
> -static void ler_enable(void)
> -{
> -    u64 debugctl;
> -
> -    if ( !this_cpu(ler_msr) )
> -        return;
> -
> -    rdmsrl(MSR_IA32_DEBUGCTLMSR, debugctl);
> -    wrmsrl(MSR_IA32_DEBUGCTLMSR, debugctl | IA32_DEBUGCTLMSR_LBR);
> -}
> -
>  void do_debug(struct cpu_user_regs *regs)
>  {
>      unsigned long dr6;
> @@ -1870,13 +1860,13 @@ void do_debug(struct cpu_user_regs *regs)
>      v->arch.debugreg[6] |= (dr6 & ~X86_DR6_DEFAULT);
>      v->arch.debugreg[6] &= (dr6 | ~X86_DR6_DEFAULT);
>  
> -    ler_enable();
>      pv_inject_hw_exception(TRAP_debug, X86_EVENT_NO_EC);
> -    return;
>  
>   out:
> -    ler_enable();
> -    return;
> +
> +    /* #DB automatically disabled LBR.  Reinstate it if debugging Xen. */
> +    if ( cpu_has_xen_lbr )
> +        wrmsrl(MSR_IA32_DEBUGCTLMSR, IA32_DEBUGCTLMSR_LBR);

While I can see that we don't currently need anything more than this one
bit, it still doesn't feel overly well to not do a read-modify-write cycle here.

In any event, rather than moving the write further towards the end of
the function, could I ask you to move it further up, so that in the (unlikely)
event of do_debug() itself triggering an exception we'd get a proper
indication of the last branch before that?

> @@ -1920,38 +1910,46 @@ void load_TR(void)
>          : "=m" (old_gdt) : "rm" (TSS_ENTRY << 3), "m" (tss_gdt) : "memory" );
>  }
>  
> -void percpu_traps_init(void)
> +static uint32_t calc_ler_msr(void)

Here and elsewhere "unsigned int" would be more appropriate to use.
We don't require MSR indexes to be exactly 32 bits wide, but only at
least as wide.

> +void percpu_traps_init(void)
> +{
> +    subarch_percpu_traps_init();
> +
> +    if ( !opt_ler )
> +        return;
> +
> +    if ( !ler_msr && (ler_msr = calc_ler_msr()) )
> +        setup_force_cpu_cap(X86_FEATURE_XEN_LBR);

This does not hold up with the promise the description makes: If running
on an unrecognized model, calc_ler_msr() is going to be called more than
once. If it really was called just once, it could also become __init. With
the inverted sense of the feature flag (as suggested above) you could
check whether the flag bit is set or ler_msr is non-zero.

Jan



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