Re: [Xen-devel] [PATCH] xen/arm: Add support for 16 bit VMIDs

[Julien Grall <julien.grall@xxxxxxx>]

Hi Stefano,

On 19/11/16 04:34, Stefano Stabellini wrote:
> On Fri, 11 Nov 2016, Bhupinder Thakur wrote:
> > VMID space is increased to 16-bits from 8-bits in ARMv8 8.1 revision.
> > This allows more than 256 VMs to be supported by Xen.
> >
> > This change adds support for 16-bit VMIDs in Xen based on whether the
> > architecture supports it.
> >
> > Signed-off-by: Bhupinder Thakur <bhupinder.thakur@xxxxxxxxxx>
[...]

> >  xen/arch/arm/p2m.c              | 44 +++++++++++++++++++++++++++++++++++------
> >  xen/include/asm-arm/p2m.h       |  2 +-
> >  xen/include/asm-arm/processor.h | 17 +++++++++++++++-
> >  3 files changed, 55 insertions(+), 8 deletions(-)
> >
> > diff --git a/xen/arch/arm/p2m.c b/xen/arch/arm/p2m.c
> > index cc5634b..6ed7e5c 100644
> > --- a/xen/arch/arm/p2m.c
> > +++ b/xen/arch/arm/p2m.c
> > @@ -19,6 +19,7 @@ static unsigned int __read_mostly p2m_root_order;
> >  static unsigned int __read_mostly p2m_root_level;
> >  #define P2M_ROOT_ORDER    p2m_root_order
> >  #define P2M_ROOT_LEVEL p2m_root_level
> > +static unsigned int __read_mostly max_vmid;
> >  #else
> >  /* First level P2M is alway 2 consecutive pages */
> >  #define P2M_ROOT_LEVEL 1
> > @@ -1219,7 +1220,7 @@ static int p2m_alloc_table(struct domain *d)
> >
> >      p2m->root = page;
> >
> > -    p2m->vttbr = page_to_maddr(p2m->root) | ((uint64_t)p2m->vmid & 0xff) << 48;
> > +    p2m->vttbr = page_to_maddr(p2m->root) | ((uint64_t)p2m->vmid << 48);
> >
> >      /*
> >       * Make sure that all TLBs corresponding to the new VMID are flushed
> > @@ -1230,20 +1231,47 @@ static int p2m_alloc_table(struct domain *d)
> >      return 0;
> >  }
> >
> > -#define MAX_VMID 256
> > +#ifdef CONFIG_ARM_64
> > +#define MAX_VMID  (1UL << 16)
> > +#else
> > +#define MAX_VMID (1UL << 8)
> > +#endif
> Given that MAX_VMID on ARM64 can be either 256 or 65536, and given that
> this patch also introduces max_vmid, I find these #defines confusing. It
> is not obvious how max_vmid and MAX_VMID differ. I would go for
> something like the following:
>
> #define MAX_VMID_8  (1UL << 8)
> #define MAX_VMID_16 (1UL << 16)
> #ifdef CONFIG_ARM_64
> #define MAX_VMID_ARCH MAX_VMID_16
> #else
> #define MAX_VMID_ARCH MAX_VMID_8
> #endif
MAX_VMID is mostly used to define the vmid at compilation time.
However, with the support of 16 bits VMID, the size is now 8KB. So we 
would increase Xen footprint by 8KB on all AArch64 platform (even non 
ARMv8.1 compliant).
I would much prefer to see this bitmap allocating at runtime. With that 
we could drop the use of MAX_VMID.
Regards,

--
Julien Grall

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