Re: [PATCH v2 1/4] x86/compat: Test whether guest has 32b shinfo instead of being a PV 32b domain

[Boris Ostrovsky <boris.ostrovsky@oracle.com>]

On 07/07/2015 12:15 PM, Jan Beulich wrote:
>>>> On 07.07.15 at 17:46, <boris.ostrovsky@oracle.com> wrote:
>> On 07/07/2015 05:11 AM, Jan Beulich wrote:
>>>>>> On 29.06.15 at 22:21, <boris.ostrovsky@oracle.com> wrote:
>>>> @@ -737,7 +737,7 @@ int arch_set_info_guest(
>>>>    
>>>>        /* The context is a compat-mode one if the target domain is compat-mode;
>>>>         * we expect the tools to DTRT even in compat-mode callers. */
>>>> -    compat = is_pv_32on64_domain(d);
>>>> +    compat = has_32bit_shinfo(d);
>>> Furthermore, looking at uses like this, tying such decisions to the
>>> shared info layout looks kind of odd. I think for documentation
>>> purposes we may need a differently named alias.
>> Yes, it does look odd, which is why I was asking in another thread about
>> having another field in domain structure (well, I was asking about
>> replacing has_32bit_shinfo but I think I can see now that wouldn't be
>> right).
>>
>> Are you suggesting a new macro, e.g.
>> #define is_32b_mode(d)    ((d)->arch.has_32bit_shinfo)
>>
>> or would it better to add new field? Or get_mode() hvm op, similar to
>> set_mode(), which can look, say, at EFER?
> If looking at EFER (plus perhaps CS) is right in all the cases you
> care about, then yes. And remember we already have
> hvm_guest_x86_mode().

Can't use hvm_guest_x86_mode(), it asserts on 'v != current'. But adding 
new op just because of that seems to be an overkill since it would 
essentially do what .guest_x86_mode() does. How about 
hvm_guest_x86_mode_unsafe() (with a better name) and wrap 
hvm_guest_x86_mode() with the ASSERT around it?

>
>>>> --- a/xen/common/domctl.c
>>>> +++ b/xen/common/domctl.c
>>>> @@ -496,7 +496,7 @@ long do_domctl(XEN_GUEST_HANDLE_PARAM(xen_domctl_t) u_domctl)
>>>>                break;
>>>>    
>>>>    #ifdef CONFIG_COMPAT
>>>> -        if ( !is_pv_32on64_domain(d) )
>>>> +        if ( !has_32bit_shinfo(d) )
>>>>                ret = copy_from_guest(c.nat, op->u.vcpucontext.ctxt, 1);
>>>>            else
>>>>                ret = copy_from_guest(c.cmp,
>>>> @@ -902,7 +902,7 @@ long do_domctl(XEN_GUEST_HANDLE_PARAM(xen_domctl_t) u_domctl)
>>>>            vcpu_unpause(v);
>>>>    
>>>>    #ifdef CONFIG_COMPAT
>>>> -        if ( !is_pv_32on64_domain(d) )
>>>> +        if ( !has_32bit_shinfo(d) )
>>>>                ret = copy_to_guest(op->u.vcpucontext.ctxt, c.nat, 1);
>>>>            else
>>>>                ret = copy_to_guest(guest_handle_cast(op->u.vcpucontext.ctxt,
>>> Where is it written down what format 32-bit PVH guests' vCPU
>>> contexts get passed in? It would seem to me that it would be
>>> rather more natural for them to use the 64-bit layout. Or else
>>> how do you intend to suppress them being able to enter 64-bit
>>> mode?
>> So why do we use the 'else' clause for 32b PV guests when they also use
>> the same vcpu_guest_context_x86_32_t in libxc/xc_dom_x86.c:vcpu_x86_32()?
> 32bit PV guests use the if() branch afaict (as they use the 32-bit
> shared info layout).

No, they use the 'else' part, I just confirmed it. 'd' in 
is_pv_32on64_domain() is domain for which domctl is being called, not 
domain that is making the call (which is what I suspect the original 
intent was).

-boris