Re: [PATCH] x86/paravirt: Fix build PARAVIRT_XXL=y without XEN_PV

[Juergen Gross]

On 19.11.21 11:20, Kirill A. Shutemov wrote:
> On Fri, Nov 19, 2021 at 03:51:44PM +0800, Lai Jiangshan wrote:
> > On Thu, Nov 18, 2021 at 7:14 AM Kirill A. Shutemov <kirill@xxxxxxxxxxxxx> wrote:
> > > TDX is going to use CONFIG_PARAVIRT_XXL, but kernel fails to compile if
> > > XEN_PV is not enabled:
> > >
> > >          ld.lld: error: undefined symbol: xen_iret
> > >
> > > It happens because INTERRUPT_RETURN defined to use xen_iret if
> > > CONFIG_PARAVIRT_XXL enabled regardless of CONFIG_XEN_PV.
> > >
> > > The issue is not visible in the current kernel because CONFIG_XEN_PV is
> > > the only user of CONFIG_PARAVIRT_XXL and there's no way to enable them
> > > separately.
> > >
> > > Rework code to define INTERRUPT_RETURN based on CONFIG_XEN_PV, not
> > > CONFIG_PARAVIRT_XXL.
> > >
> > > Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> > > Cc: Juergen Gross <jgross@xxxxxxxx>
> > > Cc: Deep Shah <sdeep@xxxxxxxxxx>
> > > Cc: "VMware, Inc." <pv-drivers@xxxxxxxxxx>
> > > ---
> > >   arch/x86/include/asm/irqflags.h | 7 +++++--
> > >   arch/x86/include/asm/paravirt.h | 5 -----
> > >   2 files changed, 5 insertions(+), 7 deletions(-)
> > >
> > > diff --git a/arch/x86/include/asm/irqflags.h b/arch/x86/include/asm/irqflags.h
> > > index b794b6da3214..3b8ddcb7be76 100644
> > > --- a/arch/x86/include/asm/irqflags.h
> > > +++ b/arch/x86/include/asm/irqflags.h
> > > @@ -118,8 +118,6 @@ static __always_inline unsigned long arch_local_irq_save(void)
> > >   #define SAVE_FLAGS             pushfq; popq %rax
> > >   #endif
> > >
> > > -#define INTERRUPT_RETURN       jmp native_iret
> > > -
> > >   #endif
> > >
> > >   #endif /* __ASSEMBLY__ */
> > > @@ -147,8 +145,13 @@ static __always_inline void arch_local_irq_restore(unsigned long flags)
> > >   #ifdef CONFIG_X86_64
> > >   #ifdef CONFIG_XEN_PV
> > >   #define SWAPGS ALTERNATIVE "swapgs", "", X86_FEATURE_XENPV
> > > +#define INTERRUPT_RETURN                                               \
> > > +       ANNOTATE_RETPOLINE_SAFE;                                        \
> > > +       ALTERNATIVE_TERNARY("jmp *paravirt_iret(%rip);",                \
> > > +               X86_FEATURE_XENPV, "jmp xen_iret;", "jmp native_iret;")
> >
> > It is part of what CONFIG_PARAVIRT_XXL was designed for to enable
> > pv-aware INTERRUPT_RETURN.
>
> That's very vague statement.
>
> Could you elaborate on what is wrong with proposed fix?
>
> > I would prefer xen_iret is defined as a weak symbol unconditionally.
> > Like:
> >
> > diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
> > index e38a4cf795d9..c0953f1b4559 100644
> > --- a/arch/x86/entry/entry_64.S
> > +++ b/arch/x86/entry/entry_64.S
> > @@ -635,6 +635,7 @@ SYM_INNER_LABEL_ALIGN(native_iret, SYM_L_GLOBAL)
> >          jnz     native_irq_return_ldt
> >   #endif
> >
> > +SYM_INNER_LABEL(xen_iret, SYM_L_WEAK) /* placeholder */
> >   SYM_INNER_LABEL(native_irq_return_iret, SYM_L_GLOBAL)
> >          /*
> >           * This may fault.  Non-paranoid faults on return to userspace are
> >
> > It will work when !CONFIG_XEN_PV
>
> It pollutes namespace for no particular reason. I don't see it justified.

I agree. IMO weak symbols should be avoided if easily possible, and the
patch here shows that it IS possible without any problem.


Juergen

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