[RFC PATCH kernel] KVM: PPC: Book3S PR: Fix WIMG handling under pHyp

[Alexander Graf]

On 24.11.17 00:33, Greg Kurz wrote:
> On Wed, 22 Nov 2017 14:42:21 +1100
> Alexey Kardashevskiy <aik at ozlabs.ru> wrote:
> 
>> 96df226 "KVM: PPC: Book3S PR: Preserve storage control bits" added WIMG
>> bits preserving but it missed 2 special cases:
>> - a magic page in kvmppc_mmu_book3s_64_xlate() and
>> - guest real mode in kvmppc_handle_pagefault().
>>
>> For these ptes WIMG were 0 and pHyp failed on these causing a guest to
>> stop in the very beginning at NIP=0x100 (due to bd9166ffe
>> "KVM: PPC: Book3S PR: Exit KVM on failed mapping").
>>
>> This initializes WIMG to non-zero value HPTE_R_M. The value is chosen
>> as (0x192 & HPTE_R_WIMG); 0x192 is a magic value from
>> kvmppc_mmu_map_page().
>>
>> Fixes: 96df226 "KVM: PPC: Book3S PR: Preserve storage control bits"
>> Signed-off-by: Alexey Kardashevskiy <aik at ozlabs.ru>
>> ---
>>
>> This indeed fixes PR KVM + VFIO under pHyp but selection of HPTE_R_M
>> is arguable.
>>
> 
> The initial page fault at 0x100 on machine startup causes H_ENTER to fail in
> pHyp with H_PARAMETER, as described in PAPR:
> 
>  The hypervisor checks that the WIMG bits within the PTE are appropriate for the
>  physical page number else H_Parameter return. (For System Memory pages WIMG=0010,
>  or, 1110 if the SAO option is enabled, and for IO pages WIMG=01**.)
> 
> I'm not aware we care for SAO so HPTE_R_M looks like the only sensible choice,
> or I'm missing something ?
> 
> Anyway, this patch allows to start a PR guest on PowerVM again.
> 
> Reviewed-by: Greg Kurz <groug at kaod.org>
> 
> and
> 
> Tested-by: Greg Kurz <groug at kaod.org>

So doesn't that mean we should pick it up, mark it CC stable and push it
upstream?


Alex