[PATCH] powerpc/64s: Fix restore of NV GPRs after facility unavailable exception

[Michael Ellerman]

On Tue, 2020-05-26 at 06:18:08 UTC, Michael Ellerman wrote:
> Commit 702f09805222 ("powerpc/64s/exception: Remove lite interrupt
> return") changed the interrupt return path to not restore non-volatile
> registers by default, and explicitly restore them in paths where it is
> required.
> 
> But it missed that the facility unavailable exception can sometimes
> modify user registers, ie. when it does emulation of move from DSCR.
> 
> This is seen as a failure of the dscr_sysfs_thread_test:
>   test: dscr_sysfs_thread_test
>   [cpu 0] User DSCR should be 1 but is 0
>   failure: dscr_sysfs_thread_test
> 
> So restore non-volatile GPRs after facility unavailable exceptions.
> 
> Currently the hypervisor facility unavailable exception is also wired
> up to call facility_unavailable_exception().
> 
> In practice we should never take a hypervisor facility unavailable
> exception for the DSCR. On older bare metal systems we set HFSCR_DSCR
> unconditionally in __init_HFSCR, or on newer systems it should be
> enabled via the "data-stream-control-register" device tree CPU
> feature.
> 
> Even if it's not, since commit f3c99f97a3cd ("KVM: PPC: Book3S HV:
> Don't access HFSCR, LPIDR or LPCR when running nested"), the KVM code
> has unconditionally set HFSCR_DSCR when running guests.
> 
> So we should only get a hypervisor facility unavailable for the DSCR
> if skiboot has disabled the "data-stream-control-register" feature,
> and we are somehow in guest context but not via KVM.
> 
> Given all that, it should be unnecessary to add a restore of
> non-volatile GPRs after the hypervisor facility exception, because we
> never expect to hit that path. But equally we may as well add the
> restore, because we never expect to hit that path, and if we ever did,
> at least we would correctly restore the registers to their post
> emulation state.
> 
> In future we can split the non-HV and HV facility unavailable handling
> so that there is no emulation in the HV handler, and then remove the
> restore for the HV case.
> 
> Fixes: 702f09805222 ("powerpc/64s/exception: Remove lite interrupt return")
> Signed-off-by: Michael Ellerman <mpe at ellerman.id.au>

Applied to powerpc fixes.

https://git.kernel.org/powerpc/c/595d153dd1022392083ac93a1550382cbee127e0

cheers