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

On 24.11.17 00:33, Greg Kurz wrote:

On Wed, 22 Nov 2017 14:42:21 +1100
Alexey Kardashevskiy [off-list ref] wrote:

quoted

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 <redacted>
---

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 <redacted>

and

Tested-by: Greg Kurz <redacted>

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


Alex