target/ppc: Fix mtmsr(d) L=1 variant that loses interrupts

If mtmsr L=1 sets MSR[EE] while there is a maskable exception pending,
it does not cause an interrupt. This causes the test case to hang:

https://lists.gnu.org/archive/html/qemu-ppc/2019-10/msg00826.html

More recently, Linux reduced the occurance of operations (e.g., rfi)
which stop translation and allow pending interrupts to be processed.
This started causing hangs in Linux boot in long-running kernel tests,
running with '-d int' shows the decrementer stops firing despite DEC
wrapping and MSR[EE]=1.

https://lists.ozlabs.org/pipermail/linuxppc-dev/2020-April/208301.html

The cause is the broken mtmsr L=1 behaviour, which is contrary to the
architecture. From Power ISA v3.0B, p.977, Move To Machine State Register,
Programming Note states:

    If MSR[EE]=0 and an External, Decrementer, or Performance Monitor
    exception is pending, executing an mtmsrd instruction that sets
    MSR[EE] to 1 will cause the interrupt to occur before the next
    instruction is executed, if no higher priority exception exists

Fix this by handling L=1 exactly the same way as L=0, modulo the MSR
bits altered.

The confusion arises from L=0 being "context synchronizing" whereas L=1
is "execution synchronizing", which is a weaker semantic. However this
is not a relaxation of the requirement that these exceptions cause
interrupts when MSR[EE]=1 (e.g., when mtmsr executes to completion as
TCG is doing here), rather it specifies how a pipelined processor can
have multiple instructions in flight where one may influence how another
behaves.

Cc: qemu-stable@nongnu.org
Reported-by: Anton Blanchard <anton@ozlabs.org>
Reported-by: Nathan Chancellor <natechancellor@gmail.com>
Tested-by: Nathan Chancellor <natechancellor@gmail.com>
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Message-Id: <20200414111131.465560-1-npiggin@gmail.com>
Reviewed-by: Cédric Le Goater <clg@kaod.org>
Tested-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

0 files changed, 0 insertions, 0 deletions