Re: [Qemu-ppc] Power9 VRMA emulation

[Amol Surati]

On Thu, May 23, 2019 at 09:32:06AM +1000, David Gibson wrote:
> On Sat, May 18, 2019 at 10:47:22AM +0530, Amol Surati wrote:
> > Hello again,
> > 
> > 
> > A bit of testing was carried out.
> > 
> > 
> > 
> > As it is now, qemu raises a HISI without consulting the VRMA SLB and the
> > partitions page table.
> > 
> > This is the message it prints for when a (non-zero LPIDR) partition was made
> > to run under VRMA (for an arbitrary instruction address 0x8ee8):
> > 
> > "Raise exception at 0000000000008ee8 => 00000046 (8000000)"
> > The error code 8000000 is SRR1_PROTFAULT.
> > 
> > 
> > 
> > After allowing LPCR_VPM0 as a valid bit for POWERPC_MMU_3_00, and setting
> > it to 1, the above test was repeated. This is the output:
> > 
> > htab_base 0000000000000000 htab_mask 00000000000007ff hash 0003fffffffffff7
> > 0 htab=0000000000000000/00000000000007ff vsid=0000000001ffffff
> > ptem=4001ffffff000001 hash=0003fffffffffff7
> > 1 htab=0000000000000000/00000000000007ff vsid=0000000001ffffff
> > api=4001ffffff000003 hash=fffc000000000008
> > Raise exception at 0000000000008ee8 => 00000046 (40000000)
> > 
> > The error code 40000000 is SRR1_NOPTE. That code and qemu's attempt to
> > search in the page table for the partition, correspond with the comment
> > in the ISA that HISI is to be raised only when (1) VPM is ON and (2)
> > conditions exist which warrant raising a ISI/DSI.
> > 
> > IIURC, the conditions which warrant raising a storage interrupt are those
> > which correspond to failure in translating VA to RA (i.e. failure in
> > finding a valid PTE, assuming TLB was empty).
> > 
> > 
> > 
> > 
> > QEMU seems to ignore that bit of ISA and raises the HISI (in the above
> > test) without checking the page table. Instead of removing LPCR_VPM0 bit
> > for POWERPC_MMU_3_00, qemu should consider that bit as 1 for all
> > purposes except for the display purpose.
> > 
> > Does that make sense, or am I misinterpreting the ISA?
> 
> Which version of qemu are you using?  I quite recently merged a bunch
> of patches from Suraj which make a number of fixes to TCG's HV mode
> implementation.

The version is the latest available at qemu's official upstream
repository - "QEMU emulator version 4.0.50 (v4.0.0-718-ga4f667b671)" at
the moment.

I also checked (but did not build and run) dgibson/master,
dgibson/ppc-for-4.1 and legoater/powernv-4.1 for how these repos treat
VPM0 and VRMA.

Although all four repos know about VPM0 being deprecated in P9, they
treat VPM0 as OFF for the P9 mmu. The reason could be that VRMA isn't
supported on P9 yet, as hinted by the comments found here:

"Re-enable RMLS on POWER9 for virtual hypervisors".
https://patchwork.kernel.org/patch/10815527/


Thank you,
Amol

> 
> -- 
> David Gibson                  | I'll have my music baroque, and my code
> david AT gibson.dropbear.id.au        | minimalist, thank you.  NOT _the_ 
> _other_
>                               | _way_ _around_!
> http://www.ozlabs.org/~dgibson