Date:   Fri, 19 Nov 2021 13:20:48 +0300
From:   "Kirill A. Shutemov" <kirill@shutemov.name>
To:     Lai Jiangshan <jiangshanlai+lkml@gmail.com>
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        Kuppuswamy Sathyanarayanan 
        <sathyanarayanan.kuppuswamy@linux.intel.com>,
        Andi Kleen <ak@linux.intel.com>,
        Dave Hansen <dave.hansen@intel.com>,
        Peter Zijlstra <peterz@infradead.org>, X86 ML <x86@kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Juergen Gross <jgross@suse.com>, Deep Shah <sdeep@vmware.com>,
        "VMware, Inc." <pv-drivers@vmware.com>
Subject: Re: [PATCH] x86/paravirt: Fix build PARAVIRT_XXL=y without XEN_PV
Message-ID: <20211119102048.hjm46edvhryjgluy@box.shutemov.name>
References: <20211117181439.4368-1-kirill.shutemov@linux.intel.com>
 <CAJhGHyBRcAg33DxxGGpbnT+O7CGKp6Ktr3PtCZ0j50oTJ4fgrg@mail.gmail.com>
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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@shutemov.name> 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@linux.intel.com>
> > Cc: Juergen Gross <jgross@suse.com>
> > Cc: Deep Shah <sdeep@vmware.com>
> > Cc: "VMware, Inc." <pv-drivers@vmware.com>
> > ---
> >  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.

-- 
 Kirill A. Shutemov