linux-kernel - Re: [PATCH v5 1/2] x86/mm: add a function to check if a pfn is UC/UC-

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Date:   Thu, 16 Nov 2017 14:13:00 +0800
From:   Haozhong Zhang <haozhong.zhang@...el.com>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     KVM list <kvm@...r.kernel.org>, X86 ML <x86@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Paolo Bonzini <pbonzini@...hat.com>, rkrcmar@...hat.com,
        Xiao Guangrong <xiaoguangrong.eric@...il.com>,
        "Cuevas Escareno, Ivan D" <ivan.d.cuevas.escareno@...el.com>,
        "Kumar, Karthik" <karthik.kumar@...el.com>,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        Olif Chapman <olif.chapman@...cle.com>,
        Mikulas Patocka <mpatocka@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, Borislav Petkov <bp@...e.de>,
        Tom Lendacky <thomas.lendacky@....com>
Subject: Re: [PATCH v5 1/2] x86/mm: add a function to check if a pfn is UC/UC-

On 11/15/17 07:17 -0800, Dan Williams wrote:
> On Tue, Nov 7, 2017 at 11:56 PM, Haozhong Zhang
> <haozhong.zhang@...el.com> wrote:
> > It will be used by KVM to check whether a pfn should be
> > mapped to guest as UC.
> >
> > Signed-off-by: Haozhong Zhang <haozhong.zhang@...el.com>
> > ---
> >  arch/x86/include/asm/pat.h |  2 ++
> >  arch/x86/mm/pat.c          | 16 ++++++++++++++++
> >  2 files changed, 18 insertions(+)
> >
> > diff --git a/arch/x86/include/asm/pat.h b/arch/x86/include/asm/pat.h
> > index fffb2794dd89..fabb0cf00e77 100644
> > --- a/arch/x86/include/asm/pat.h
> > +++ b/arch/x86/include/asm/pat.h
> > @@ -21,4 +21,6 @@ int io_reserve_memtype(resource_size_t start, resource_size_t end,
> >
> >  void io_free_memtype(resource_size_t start, resource_size_t end);
> >
> > +bool pat_pfn_is_uc_or_uc_minus(unsigned long pfn);
> > +
> >  #endif /* _ASM_X86_PAT_H */
> > diff --git a/arch/x86/mm/pat.c b/arch/x86/mm/pat.c
> > index fe7d57a8fb60..e1282dd4eeb8 100644
> > --- a/arch/x86/mm/pat.c
> > +++ b/arch/x86/mm/pat.c
> > @@ -677,6 +677,22 @@ static enum page_cache_mode lookup_memtype(u64 paddr)
> >         return rettype;
> >  }
> >
> > +/**
> > + * Check with PAT whether the memory type of a pfn is UC or UC-.
> > + *
> > + * Only to be called when PAT is enabled.
> > + *
> > + * Returns true, if the memory type of @pfn is UC or UC-.
> > + * Otherwise, returns false.
> > + */
> > +bool pat_pfn_is_uc_or_uc_minus(unsigned long pfn)
> > +{
> > +       enum page_cache_mode cm = lookup_memtype(PFN_PHYS(pfn));
> > +
> > +       return cm == _PAGE_CACHE_MODE_UC || cm == _PAGE_CACHE_MODE_UC_MINUS;
> > +}
> > +EXPORT_SYMBOL_GPL(pat_pfn_is_uc_or_uc_minus);
> 
> Why do we need this strangely named new accessor? It seems to be
> open-coding a new / more limited version of track_pfn_insert().

In the first version patchset, KVM did extract and check the cache
mode got from track_pfn_insert(), but Ingo thought it was better to
keep all the low-level details out of KVM, so I encapsulated them in a
function in mm in subsequent versions.

Haozhong