Re: [Xen-devel] Is: axe read_tscp pvops call. Was: Re: [RFC] ACPI S3 and Xen (suprisingly small\!).

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

On Wed, Oct 17, 2012 at 12:10:36PM -0400, Konrad Rzeszutek Wilk wrote:
> On Wed, Oct 17, 2012 at 09:03:12AM -0700, H. Peter Anvin wrote:
> > On 10/17/2012 06:49 AM, Konrad Rzeszutek Wilk wrote:
> > >
> > >Note: These are the other patches that went in 3.7-rc1:
> > >xen/bootup: allow {read|write}_cr8 pvops call 
> > >[https://lkml.org/lkml/2012/10/10/339]
> > >xen/bootup: allow read_tscp call for Xen PV guests. 
> > >[https://lkml.org/lkml/2012/10/10/340]
> > >
> > 
> > So WTF do we have a read_tscp PV call?  Again, if there isn't a user
> > we should just axe it...
> 
> Let me spin off a patch to see if that can be done.

It can be done faily easy. That said  the only user that could
_potentially_ use this (if the read_tscp had some extra logic to
do 'readtsc' operations) would be the __vdso_getcpu.

Meaning in __vdso_getcpu we would modify it from native_read_tscp
to paravirt_read_tscp:

notrace long
__vdso_getcpu(unsigned *cpu, unsigned *node, struct getcpu_cache *unused)
{
        unsigned int p;

        if (VVAR(vgetcpu_mode) == VGETCPU_RDTSCP) {
                /* Load per CPU data from RDTSCP */
===>            native_read_tscp(&p);
        } else {
                /* Load per CPU data from GDT */
                asm("lsl %1,%0" : "=r" (p) : "r" (__PER_CPU_SEG));
        }
        if (cpu)
                *cpu = p & 0xfff;
        if (node)
                *node = p >> 12;
        return 0;
}

but that line was added for a purpose, which was
in git commit 8f12dea6135d0a55b151dcb4c6bbe211f5f8d35d
Author: Glauber de Oliveira Costa <gcosta@xxxxxxxxxx>
Date:   Wed Jan 30 13:31:06 2008 +0100

    x86: introduce native_read_tscp
    
    Targetting paravirt, this patch introduces native_read_tscp, in
    place of rdtscp() macro. When in a paravirt guest, this will
    involve a function call, and thus, cannot be done in the vdso area.
    These users then have to call the native version directly
    
    Signed-off-by: Glauber de Oliveira Costa <gcosta@xxxxxxxxxx>
    Signed-off-by: Ingo Molnar <mingo@xxxxxxx>
    Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>

which implies that it since it is a vDSO area it cannot do paravirt
calls anyhow.

In other words, I think I'm OK with axing it. Going to spin a patch
and ask for some other folks to review/double check.

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