RE: [Xen-devel] 32/64-bit hypercall interface

["Nakajima, Jun" <jun.nakajima@xxxxxxxxx>]

Hollis Blanchard wrote:
> On Monday 03 October 2005 16:24, Nakajima, Jun wrote:
>> Hollis Blanchard wrote:
>>> As mentioned previously, this is the approach Linux uses
>>> (linux/fs/compat_ioctl.c), and it seems less than ideal to me. Since
>>> we have the ability to fix it now (i.e. make the 32-bit and 64-bit
>>> ABI identical), shouldn't we do that rather than this
>>> copying/munging layer?
>> 
>> The 32-bit and 64-bit hypercall ABI cannot be identical on x86
>> because of the generic ABI difference between 32-bit and 64-bit.
> 
> I am not talking about the standard ABI used by the compiler (ppc32
> and ppc64 use different ABIs as well). I am talking about the
> hypervisor/tools and hypervisor/kernel ABI.
> 
> If the hypervisor ABI does not contain types that change size, it
> will be identical for both 32- and 64-bit users.
> 
Okay, I understand what you need. How about the data layout? That's not
so trivial to have the identical ones. 
For example, the following one would have different data layout on
32-bit and 64-bit. 

typedef struct vcpu_time_info {
    /*
     * Updates to the following values are preceded and followed by an
     * increment of 'version'. The guest can therefore detect updates by
     * looking for changes to 'version'. If the least-significant bit of
     * the version number is set then an update is in progress and the
guest
     * must wait to read a consistent set of values.
     * The correct way to interact with the version number is similar to
     * Linux's seqlock: see the implementations of
read_seqbegin/read_seqretry.
     */
    u32 version;
    u64 tsc_timestamp;   /* TSC at last update of time vals.  */
    u64 system_time;     /* Time, in nanosecs, since boot.    */
    /*
     * Current system time:
     *   system_time + ((tsc - tsc_timestamp) << tsc_shift) *
tsc_to_system_mul
     * CPU frequency (Hz):
     *   ((10^9 << 32) / tsc_to_system_mul) >> tsc_shift
     */
    u32 tsc_to_system_mul;
    s8  tsc_shift;
} vcpu_time_info_t;


> If you meant something else when you said "generic ABI difference,"
> could you explain?

I meant basically the "standard ABI", such as, parameter passing, i.e.
which registers are used to pass which arguments. x86_64 xenlinux runs
in ring3, and hypercalls are not so different from system calls in terms
of the mechanism/implementation. Then when we use fast system calls, we
can use the compiler convention to minimize save/restore of the
registers.

Jun
---
Intel Open Source Technology Center

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