[RFC] arm64: defconfig: enable 48-bit VA by default

[Stuart Yoder]

> -----Original Message-----
> From: Marc Zyngier [mailto:marc.zyngier at arm.com]
> Sent: Thursday, July 23, 2015 7:44 AM
> To: Yoder Stuart-B08248; Catalin Marinas; Will Deacon
> Cc: linux-arm-kernel at lists.infradead.org; Newton Peter-RA3823
> Subject: Re: [RFC] arm64: defconfig: enable 48-bit VA by default
> 
> On 22/07/15 20:49, Stuart Yoder wrote:
> > Catalin/Will,
> >
> > This is not a patch mean to be applied, but a query about whether there
> > is any reason to not enable 48-bit VA by default in the arm64 defconfig.
> >
> > The Freescale LS2085A physical memory map requires 48-bit VA in Linux for the
> > reasons mentioned in [1].
> >
> > Based on the comment in [1] by Catalin, it seems that the intent
> > is to turn this on by default.
> >
> > Is there any issues anyone sees with a patch that does this:
> >
> > diff --git a/arch/arm64/configs/defconfig b/arch/arm64/configs/defconfig
> > index 4e17e7e..5acf75d 100644
> > @@ -47,6 +47,7 @@ CONFIG_ARCH_ZYNQMP=y
> >  CONFIG_PCI=y
> >  CONFIG_PCI_MSI=y
> >  CONFIG_PCI_XGENE=y
> > +CONFIG_ARM64_VA_BITS_48=y
> >  CONFIG_SMP=y
> >  CONFIG_PREEMPT=y
> >  CONFIG_KSM=y
> >
> > Thanks,
> > Stuart
> >
> > [1] https://www.marc.info/?l=linux-arm-kernel&m=140965303205473&w=1
> >
> 
> Is that still a requirement now that our idmap can use 4 levels (as part
> of dd006da)?

So, yes it appears still to be a requirement.  The idmap support is not
the issue, it's the linear mapping.  

Has there been discussion or thinking about enabling 48-bit VA in the
default defconfig?  As mentioned before, it seemed that supporting 48-bit
VA was the planned default (~1 year ago), and was waiting on KVM issues to get
resolved.

A related question is what the thinking around enabling 64KB pages 
by default.  Any chance of that happening?

I would like to see our platform work with the default defconfig, which
is the reason for the questions.

Thanks,
Stuart