Re: [Xen-devel] [PATCH 06/11] x86: Reorganise and rename debug register fields in struct vcpu

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 04.06.18 at 15:59, <andrew.cooper3@xxxxxxxxxx> wrote:
> @@ -981,9 +981,14 @@ int arch_set_info_guest(
>      v->arch.pv_vcpu.ctrlreg[4] = cr4 ? pv_guest_cr4_fixup(v, cr4) :
>          real_cr4_to_pv_guest_cr4(mmu_cr4_features);
>  
> -    memset(v->arch.debugreg, 0, sizeof(v->arch.debugreg));
> -    for ( i = 0; i < 8; i++ )
> -        (void)set_debugreg(v, i, c(debugreg[i]));
> +    for ( i = 0; !rc && i < ARRAY_SIZE(v->arch.dr); i++ )
> +        rc = set_debugreg(v, i, c(debugreg[i]));
> +    if ( !rc )
> +        rc = set_debugreg(v, 6, c(debugreg[6]));
> +    if ( !rc )
> +        rc = set_debugreg(v, 7, c(debugreg[7]));
> +    if ( rc )
> +        return rc;

There is certainly a good intention behind this change, but it treats
one problem for another: The function has a pre-existing partial-
update-and-then-fail problem, which you now widen. Proper
behavior would imo be to never update any state when returning
an error, at least as far as anything ahead of

>      if ( v->is_initialised )
>          goto out;

goes (for not yet initialized vCPU-s the function would need to be
called again anyway before the vCPU could be started). On the
whole, until said problem gets addressed, I think I'd prefer original
behavior over the new one you switch to.

> --- a/xen/arch/x86/traps.c
> +++ b/xen/arch/x86/traps.c
> @@ -1861,8 +1861,8 @@ void do_debug(struct cpu_user_regs *regs)
>      }
>  
>      /* Save debug status register where guest OS can peek at it */
> -    v->arch.debugreg[6] |= (dr6 & ~X86_DR6_DEFAULT);
> -    v->arch.debugreg[6] &= (dr6 | ~X86_DR6_DEFAULT);
> +    v->arch.dr6 |= (dr6 & ~X86_DR6_DEFAULT);
> +    v->arch.dr6 &= (dr6 | ~X86_DR6_DEFAULT);

Shouldn't this have been changed by patch 4?

> @@ -518,7 +524,10 @@ struct arch_vcpu
>      void              *fpu_ctxt;
>      unsigned long      vgc_flags;
>      struct cpu_user_regs user_regs;
> -    unsigned long      debugreg[8];
> +
> +    /* Debug registers. */
> +    unsigned long dr[4];
> +    unsigned long dr6, dr7;

Since you make the last two separate fields, and since their upper
32 bits are reserved-zero, why not make them uint32_t, just like
dr7_emul is?

Jan



_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxxxxxxxxx
https://lists.xenproject.org/mailman/listinfo/xen-devel