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/[email protected]/ 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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