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

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 06/06/18 16:00, Jan Beulich wrote:
>>>> 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.

Hmm ok.

>
>> --- 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?

This gets fixed by patch 9.

>
>> @@ -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?

Because __vmread() takes unsigned long *.

I did initially make them uint32_t, then had the compiler complain at
me.  I indented to leave a note in the commit message, but I forgot.  Sorry.

~Andrew

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