Thomas Monjalon wrote: > From: Thomas Monjalon <tho...@monjalon.net> > > Since commit 2ada0ed, "Return From Interrupt" is broken for PPC processors > because some interrupt specifics bits of SRR1 are copied to MSR. > > SRR1 is a save of MSR during interrupt. > During RFI, MSR must be restored from SRR1. > But some bits of SRR1 are interrupt-specific and are not used for MSR saving. > > This is the specification (ISA 2.06) at chapter 6.4.3 (Interrupt Processing): > "2. Bits 33:36 and 42:47 of SRR1 or HSRR1 are loaded with information specific > to the interrupt type. > 3. Bits 0:32, 37:41, and 48:63 of SRR1 or HSRR1 are loaded with a copy of the > corresponding bits of the MSR." > > Below is a representation of MSR bits which are not saved: > 0:15 16:31 32 33:36 37:41 42:47 48:63 > ——— | ——— | — X X X X — — — — — X X X X X X | ———— > 0000 0000 | 7 | 8 | 3 | F | 0000 > > History: > In the initial Qemu implementation (e1833e1), the mask 0x783F0000 was used for > saving MSR in SRR1. But all the bits 32:47 were cleared during RFI restoring. > This was wrong. The commit 2ada0ed explains that this breaks Altivec. > Indeed, bit 38 (for Altivec support) must be saved and restored. > The change of 2ada0ed was to restore all the bits of SRR1 to MSR. > But it's also wrong. > > Explanation: > As an example, let's see what's happening after a TLB miss. > According to the e300 manual (E300CORERM table 5-6), the TLB miss interrupts > set the bits 44-47 for KEY, I/D, WAY and S/L. These bits are specifics to the > interrupt and must not be copied into MSR at the end of the interrupt. > With the current implementation, a TLB miss overwrite bits POW, TGPR and ILE. > > Fix: > It shouldn't be needed to filter-out bits on MSR saving when interrupt occurs. > Specific bits overwrite MSR ones in SRR1. > But at the end of interrupt (RFI), specifics bits must be cleared before > restoring MSR from SRR1. The mask 0x783F0000 apply here. > > Discussion: > The bits of the mask 0x783F0000 are cleared after an interrupt. > I cannot find a specification which talks about this > but I assume it is the truth since Linux can run this way. > Maybe it's not perfect but it's better (works for e300). > > Signed-off-by: Thomas Monjalon <tho...@monjalon.net> > Cc: Alexander Graf <ag...@suse.de> >
Acked-by: Alexander Graf <ag...@suse.de>
