linux-kernel - Re: [PATCH kernel 1/3] x86/amd/dr_addr_mask: Cache values in percpu variables

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Date:   Tue, 6 Dec 2022 17:07:58 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Alexey Kardashevskiy <aik@....com>
Cc:     kvm@...r.kernel.org, x86@...nel.org, linux-kernel@...r.kernel.org,
        Venu Busireddy <venu.busireddy@...cle.com>,
        Tony Luck <tony.luck@...el.com>,
        Tom Lendacky <thomas.lendacky@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Michael Sterritt <sterritt@...gle.com>,
        Michael Roth <michael.roth@....com>,
        Mario Limonciello <mario.limonciello@....com>,
        Ingo Molnar <mingo@...hat.com>,
        Heiko Carstens <hca@...ux.ibm.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Borislav Petkov <bp@...en8.de>,
        Andrew Cooper <andrew.cooper3@...rix.com>,
        "Jason A. Donenfeld" <Jason@...c4.com>,
        "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH kernel 1/3] x86/amd/dr_addr_mask: Cache values in percpu
 variables

On Tue, Dec 06, 2022, Alexey Kardashevskiy wrote:
> 
> On 2/12/22 03:58, Sean Christopherson wrote:
> > On Thu, Dec 01, 2022, Alexey Kardashevskiy wrote:
> > > diff --git a/arch/x86/include/asm/debugreg.h b/arch/x86/include/asm/debugreg.h
> > > index cfdf307ddc01..c4324d0205b5 100644
> > > --- a/arch/x86/include/asm/debugreg.h
> > > +++ b/arch/x86/include/asm/debugreg.h
> > > @@ -127,6 +127,7 @@ static __always_inline void local_db_restore(unsigned long dr7)
> > >   #ifdef CONFIG_CPU_SUP_AMD
> > >   extern void set_dr_addr_mask(unsigned long mask, int dr);
> > > +extern unsigned long get_dr_addr_mask(int dr);
> > >   #else
> > >   static inline void set_dr_addr_mask(unsigned long mask, int dr) { }
> > 
> > KVM_AMD doesn't depend on CPU_SUP_AMD, i.e. this needs a stub.  Or we need to add
> > a dependency.
> 
> The new symbol is under CONFIG_CPU_SUP_AMD and so is its only user
> arch/x86/kernel/cpu/amd.c:
> 
> arch/x86/kernel/cpu/Makefile:
> obj-$(CONFIG_CPU_SUP_AMD)               += amd.o
> 
> 
> Is this enough dependency or we need something else? (if enough, I'll put it
> into the next commit log).

No, it's actually the opposite, the issue is precisely that the symbol is buried
under CPU_SUP_AMD.  KVM_AMD doesn't currently depend on CPU_SUP_AMD, and so the
usage in sev_es_prepare_switch_to_guest() fails to compile if CPU_SUP_AMD=n and
KVM_AMD={Y,M}.

  config KVM_AMD
	tristate "KVM for AMD processors support"
	depends on KVM

I actually just submitted a patch[*] to "fix" that since KVM requires the CPU vendor
to be AMD or Hygon at runtime.  Although that patch is buried in the middle of a
large series, it doesn't have any dependencies.  So, if this series is routed through
the KVM tree, it should be straightforward to just pull that patch into this series,
and whichever series lands first gets the honor of applying that patch.

If this series is routed through the tip tree, the best option may be to just add
a stub to avoid potential conflicts, and then we can rip the stub out later.

[*] https://lore.kernel.org/all/20221201232655.290720-12-seanjc@google.com