Re: [Qemu-devel] [PATCH v3 10/20] kvm: arm64: Dynamic configuration of VTTBR mask

[Suzuki K Poulose]

Hi Eric,

On 02/07/18 15:41, Auger Eric wrote:
> Hi Suzuki,
>
> On 06/29/2018 01:15 PM, Suzuki K Poulose wrote:
> > On arm64 VTTBR_EL2:BADDR holds the base address for the stage2
> > translation table. The Arm ARM mandates that the bits BADDR[x-1:0]
> > should be 0, where 'x' is defined for a given IPA Size and the
> > number of levels for a translation granule size. It is defined
> > using some magical constants. This patch is a reverse engineered
> > implementation to calculate the 'x' at runtime for a given ipa and
> > number of page table levels. See patch for more details.
> >
> > Cc: Marc Zyngier <address@hidden>
> > Cc: Christoffer Dall <address@hidden>
> > Signed-off-by: Suzuki K Poulose <address@hidden>
> > ---
> > Changes since V2:
> >   - Part 1 of spilt from VTCR & VTTBR dynamic configuration
> > ---
> >   arch/arm64/include/asm/kvm_arm.h | 60 +++++++++++++++++++++++++++++++++++++---
> >   arch/arm64/include/asm/kvm_mmu.h | 25 ++++++++++++++++-
> >   2 files changed, 80 insertions(+), 5 deletions(-)
> >
> > diff --git a/arch/arm64/include/asm/kvm_arm.h b/arch/arm64/include/asm/kvm_arm.h
> > index 3dffd38..c557f45 100644
> > --- a/arch/arm64/include/asm/kvm_arm.h
> > +++ b/arch/arm64/include/asm/kvm_arm.h
> > @@ -140,8 +140,6 @@
> >    * Note that when using 4K pages, we concatenate two first level page tables
> >    * together. With 16K pages, we concatenate 16 first level page tables.
> >    *
> > - * The magic numbers used for VTTBR_X in this patch can be found in Tables
> > - * D4-23 and D4-25 in ARM DDI 0487A.b.
> Isn't it a pretty old reference? Could you refer to C.a?

Sure, I will update the references everywhere.

> > + *
> > + * The algorithm defines the expectations on the BaseAddress (for the page
> > + * table) bits resolved at each level based on the page size, entry level
> > + * and T0SZ. The variable "x" in the algorithm also affects the VTTBR:BADDR
> > + * for stage2 page table.
> > + *
> > + * The value of "x" is calculated as :
> > + *     x = Magic_N - T0SZ
> > + *
> > + * where Magic_N is an integer depending on the page size and the entry
> > + * level of the page table as below:
> > + *
> > + *     --------------------------------------------
> > + *     | Entry level           |  4K    16K   64K |
> > + *     --------------------------------------------
> > + *     | Level: 0 (4 levels)   | 28   |  -  |  -  |
> > + *     --------------------------------------------
> > + *     | Level: 1 (3 levels)   | 37   | 31  | 25  |
> > + *     --------------------------------------------
> > + *     | Level: 2 (2 levels)   | 46   | 42  | 38  |
> > + *     --------------------------------------------
> > + *     | Level: 3 (1 level)    | -    | 53  | 51  |
> > + *     --------------------------------------------
> I understand entry level = Lookup level in the table.

Entry level => The level at which we start the page table walk for
a given address (This is in line with the ARM ARM). So,

Entry_level = (4 - Number_of_Page_table_levels)

> But you may want to compute x for BaseAddress matching lookup level 2
> with number of levels = 4.

No, the BaseAddress is only calcualted for the "Entry_level". So the
above case doesn't exist at all.

> So shouldn't you s/Number of levels/4 - entry_level?

Ok, I now understood what you are referring to [0]
> for BADDR we want the BaseAddr of the initial lookup level so
> effectively the entry level we are interested in is 4 - number of levels
> and we don't care or d) condition. At least this is my understanding ;-)
> If correct you may slightly reword the explanation?


> > + *
> > + * We have a magic formula for the Magic_N below.
> > + *
> > + *  Magic_N(PAGE_SIZE, Entry_Level) = 64 - ((PAGE_SHIFT - 3) * Number of 
> > levels)

[0] ^^^



> > + *
> > + * where number of levels = (4 - Entry_Level).

^^^ Doesn't this help make it clear ? Using the expansion makes it a bit more
unreadable below.

> >   
> > +/*
> > + * Get the magic number 'x' for VTTBR:BADDR of this KVM instance.
> > + * With v8.2 LVA extensions, 'x' should be a minimum of 6 with
> > + * 52bit IPS.
> Link to the spec?

Sure, will add it.

Thanks for the patience to review :-)

Cheers
Suzuki