Re: [PATCH 1/2] x86: correct is_pv_domain() when !CONFIG_PV

[Jan Beulich <jbeulich@suse.com>]

On 12.04.2021 11:34, Roger Pau Monné wrote:
> On Fri, Nov 27, 2020 at 05:54:57PM +0100, Jan Beulich wrote:
>> --- a/xen/arch/x86/dom0_build.c
>> +++ b/xen/arch/x86/dom0_build.c
>> @@ -568,7 +568,7 @@ int __init construct_dom0(struct domain
>>  
>>      if ( is_hvm_domain(d) )
>>          rc = dom0_construct_pvh(d, image, image_headroom, initrd, cmdline);
>> -    else if ( is_pv_domain(d) )
>> +    else if ( is_pv_64bit_domain(d) || is_pv_32bit_domain(d) )
> 
> Urg, that's very confusing IMO, as I'm sure I would ask someone to
> just use is_pv_domain without realizing. It needs at least a comment,
> but even then I'm not sure I like it.

I can add a comment, sure, but I think this is as confusing (or not)
as ...

> So that I understand it, the point to use those expressions instead of
> is_pv_domain is to avoid calling dom0_construct_pv when CONFIG_PV is
> not enabled?
> 
> Maybe it wold be better to instead use:
> 
> if ( IS_ENABLED(CONFIG_PV) && is_pv_domain(d) )

... this.

> In any case I wonder if we should maybe aim to introduce a new type
> for system domains, that's neither PV or HVM, in order to avoid having
> system domains qualified as PV even when PV is compiled out.

This was my first thought, too, but would come with a much higher
price tag: We'd need to audit all uses for whether they're meant
to include the special domains. And this includes auditing of cases
where !is_hvm_*() may be inferred to mean is_pv_*().

>> --- a/xen/arch/x86/domain.c
>> +++ b/xen/arch/x86/domain.c
>> @@ -1544,6 +1544,7 @@ arch_do_vcpu_op(
>>   */
>>  static void load_segments(struct vcpu *n)
>>  {
>> +#ifdef CONFIG_PV
>>      struct cpu_user_regs *uregs = &n->arch.user_regs;
>>      unsigned long gsb = 0, gss = 0;
>>      bool compat = is_pv_32bit_vcpu(n);
>> @@ -1709,6 +1710,7 @@ static void load_segments(struct vcpu *n
>>          regs->cs            = FLAT_KERNEL_CS;
>>          regs->rip           = pv->failsafe_callback_eip;
>>      }
>> +#endif
>>  }
>>  
>>  /*
>> @@ -1723,6 +1725,7 @@ static void load_segments(struct vcpu *n
>>   */
>>  static void save_segments(struct vcpu *v)
>>  {
>> +#ifdef CONFIG_PV
>>      struct cpu_user_regs *regs = &v->arch.user_regs;
>>  
>>      read_sregs(regs);
>> @@ -1748,6 +1751,7 @@ static void save_segments(struct vcpu *v
>>          else
>>              v->arch.pv.gs_base_user = gs_base;
>>      }
>> +#endif
>>  }
> 
> Could you move {load,save}_segments to pv/domain.c and rename to
> pv_{load,save}_segments and provide a dummy handler for !CONFIG_PV in
> pv/domain.h?
> 
> Sorry it's slightly more work, but I think it's cleaner overall.

Doing so was my first thought too, but we'd lose the present inlining
of the functions. For save_segments() this could be dealt with by
moving paravirt_ctxt_switch_from() as well, but load_segments() would
remain.

As an aside, I've long been wondering why we use
paravirt_ctxt_switch_{from,to}() also for the idle domain. This
presently prevents their movement to pv/domain.c. From my not overly
detailed looking at it, I don't think anything the functions do
actually applies to idle vcpus.

>> --- a/xen/include/xen/sched.h
>> +++ b/xen/include/xen/sched.h
>> @@ -985,7 +985,7 @@ static always_inline bool is_control_dom
>>  
>>  static always_inline bool is_pv_domain(const struct domain *d)
>>  {
>> -    return IS_ENABLED(CONFIG_PV) &&
>> +    return IS_ENABLED(CONFIG_X86) &&
>>          evaluate_nospec(!(d->options & XEN_DOMCTL_CDF_hvm));
>>  }
>>  
>> @@ -1011,7 +1011,7 @@ static always_inline bool is_pv_32bit_vc
>>  
>>  static always_inline bool is_pv_64bit_domain(const struct domain *d)
>>  {
>> -    if ( !is_pv_domain(d) )
>> +    if ( !IS_ENABLED(CONFIG_PV) || !is_pv_domain(d) )
>>          return false;
> 
> I think overall is confusing to have a domain that returns true for
> is_pv_domain but false for both is_pv_{64,32}bit_domain checks.
> 
> I know those are only the system domains, but it feels confusing and
> could cause mistakes in the future IMO, as then we would have to
> carefully think where to use ( is_pv_64bit_domain(d)
> || is_pv_32bit_domain(d) ) vs just using is_pv_domain(d), or
> IS_ENABLED(CONFIG_PV) && is_pv_domain(d)

Imo it's not "then we would have to carefully think where to use ..."
but instead this patch is an indication that we should have been for
quite some time. For this reason (coming back to your first comment
at the top) I'm not sure adding a comment _there_ is actually useful.
Every use of is_pv_*() needs carefully considering which domains are
really meant.

Jan