Re: [PATCH v2 11/12] x86/paravirt: Don't use pv_ops vector for MSR access functions

[Peter Zijlstra <peterz@xxxxxxxxxxxxx>]

On Tue, Sep 30, 2025 at 11:02:52AM +0200, Jürgen Groß wrote:
> On 30.09.25 10:38, Peter Zijlstra wrote:
> > On Tue, Sep 30, 2025 at 09:03:55AM +0200, Juergen Gross wrote:
> > 
> > > +static __always_inline u64 read_msr(u32 msr)
> > > +{
> > > + if (cpu_feature_enabled(X86_FEATURE_XENPV))
> > > +         return xen_read_msr(msr);
> > > +
> > > + return native_rdmsrq(msr);
> > > +}
> > > +
> > > +static __always_inline int read_msr_safe(u32 msr, u64 *p)
> > > +{
> > > + if (cpu_feature_enabled(X86_FEATURE_XENPV))
> > > +         return xen_read_msr_safe(msr, p);
> > > +
> > > + return native_read_msr_safe(msr, p);
> > > +}
> > > +
> > > +static __always_inline void write_msr(u32 msr, u64 val)
> > > +{
> > > + if (cpu_feature_enabled(X86_FEATURE_XENPV))
> > > +         xen_write_msr(msr, val);
> > > + else
> > > +         native_wrmsrq(msr, val);
> > > +}
> > > +
> > > +static __always_inline int write_msr_safe(u32 msr, u64 val)
> > > +{
> > > + if (cpu_feature_enabled(X86_FEATURE_XENPV))
> > > +         return xen_write_msr_safe(msr, val);
> > > +
> > > + return native_write_msr_safe(msr, val);
> > > +}
> > > +
> > > +static __always_inline u64 rdpmc(int counter)
> > > +{
> > > + if (cpu_feature_enabled(X86_FEATURE_XENPV))
> > > +         return xen_read_pmc(counter);
> > > +
> > > + return native_read_pmc(counter);
> > > +}
> > 
> > Egads, didn't we just construct giant ALTERNATIVE()s for the native_
> > things? Why wrap that in a cpu_feature_enabled() instead of just adding
> > one more case to the ALTERNATIVE() ?
> 
> The problem I encountered with using pv_ops was to implement the *_safe()
> variants. There is no simple way to do that using ALTERNATIVE_<n>(), as
> in the Xen PV case the call will remain, and I didn't find a way to
> specify a sane interface between the call-site and the called Xen function
> to return the error indicator. Remember that at the call site the main
> interface is the one of the RDMSR/WRMSR instructions. They lack an error
> indicator.

Would've been useful Changelog material that I suppose.

> In Xin's series there was a patch written initially by you to solve such
> a problem by adding the _ASM_EXTABLE_FUNC_REWIND() exception table method.
> I think this is a dead end, as it will break when using a shadow stack.

No memories, let me go search. I found this:

  
https://patchwork.ozlabs.org/project/linux-ide/patch/20250331082251.3171276-12-xin@xxxxxxxxx/

That's the other Peter :-)

Anyway, with shadowstack you should be able to frob SSP along with SP in
the exception context. IIRC the SSP 'return' value is on the SS itself,
so a WRSS to that field can easily make the whole CALL go away.

> Additionally I found a rather ugly hack only to avoid re-iterating most of
> the bare metal ALTERNATIVE() for the paravirt case. It is possible, but the
> bare metal case is gaining one additional ALTERNATIVE level, resulting in
> patching the original instruction with an identical copy first.

OTOH the above generates atrocious crap code :/

You get that _static_cpu_has() crud, which is basically a really fat
jump_label (because it needs to include the runtime test) and then the
code for both your xen thing and the alternative.

/me ponders things a bit..

> Remember that at the call site the main interface is the one of the
> RDMSR/WRMSR instructions. They lack an error indicator.

This, that isn't true.

Note how ex_handler_msr() takes a reg argument and how that sets that
reg to -EIO. See how the current native_read_msr_safe() uses that:

  _ASM_EXTABLE_TYPE_REG(1b, 2b, EX_TYPE_RDMSR_SAFE, %[err])

(also note how using _ASM_EXTABLE_TYPE(1b, 2b, EX_TYPE_*_SAFE) like you
do, will result in reg being 0 or ax. Scribbling your 0 return value)

It very explicitly uses @err as error return value. So your call would
return eax:edx and take ecx to be the msr, but there is nothing stopping
us from then using say ebx for error return, like:

        int err = 0;

        asm_inline(
                "1:\n"
                ALTERNATIVE("ds rdmsr",
                            "call xen_rdmsr", XENPV)
                "2:\n"

                _ASM_EXTABLE_TYPE_REG(1b, 2b, EX_TYPE_RDMSR_SAFE, %%ebx)

                : "a" (ax), "d" (dx), "+b" (err)
                : "c" (msr));

        return err;

Hmm?

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