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    SubjectRe: [PATCH] KVM: arm64: fix misleading comments in save/restore
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    Marc Zyngier <marc.zyngier@arm.com> writes:

    > On 04/06/15 10:34, Christoffer Dall wrote:
    >> On Thu, May 28, 2015 at 10:43:06AM +0100, Alex Bennée wrote:
    >>> The elr_el2 and spsr_el2 registers in fact contain the processor state
    >>> before entry into the hypervisor code.
    >>
    >> be careful with your use of the hypervisor, in the KVM design the
    >> hypervisor is split across EL1 and EL2.

    "before entry into EL2."

    >>
    >>> In the case of guest state it
    >>> could be in either el0 or el1.
    >>
    >> true
    >>
    >>>
    >>> Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
    >>> ---
    >>> arch/arm64/kvm/hyp.S | 8 ++++----
    >>> 1 file changed, 4 insertions(+), 4 deletions(-)
    >>>
    >>> diff --git a/arch/arm64/kvm/hyp.S b/arch/arm64/kvm/hyp.S
    >>> index d755922..1940a4c 100644
    >>> --- a/arch/arm64/kvm/hyp.S
    >>> +++ b/arch/arm64/kvm/hyp.S
    >>> @@ -50,8 +50,8 @@
    >>> stp x29, lr, [x3, #80]
    >>>
    >>> mrs x19, sp_el0
    >>> - mrs x20, elr_el2 // EL1 PC
    >>> - mrs x21, spsr_el2 // EL1 pstate
    >>> + mrs x20, elr_el2 // PC before hyp entry
    >>> + mrs x21, spsr_el2 // pstate before hyp entry
    >>>
    >>> stp x19, x20, [x3, #96]
    >>> str x21, [x3, #112]
    >>> @@ -82,8 +82,8 @@
    >>> ldr x21, [x3, #16]
    >>>
    >>> msr sp_el0, x19
    >>> - msr elr_el2, x20 // EL1 PC
    >>> - msr spsr_el2, x21 // EL1 pstate
    >>> + msr elr_el2, x20 // PC to restore
    >>> + msr spsr_el2, x21 // pstate to restore
    >>
    >> I don't feel like 'to restore' is much more meaningful here.
    >>
    >> I would actually vote for removin the comments all together, since one
    >> should really understand the code as opposed to the comments when
    >> reading this kind of stuff.
    >>
    >> Meh, I'm not sure. Your patch is definitely better than doing nothing.
    >>
    >> Marc?
    >
    > While I definitely agree that people should pay more attention to the
    > code rather than blindly trusting comments, I still think there is some
    > value in disambiguating the exception entry/return, because this bit of
    > code assumes some intimate knowledge of the ARMv8 exception model.
    >
    > As for the comments themselves, I'd rather have some wording that
    > clearly indicate that we're dealing with guest information, i.e:
    >
    > mrs x20, elr_el2 // Guest PC
    > mrs x21, spsr_el2 // Guest pstate
    >
    > (and the same for the exception return). The "before hyp entry" and "to
    > restore" are not really useful (all the registers we are
    > saving/restoring fall into these categories). What I wanted to convey
    > here was that despite using an EL2 register, we are dealing with guest
    > registers.

    Which would be great it we were. However the code is used to
    save/restore the host context as well as the guest context hence my
    weasely words.

    >
    > Would this address your concerns?
    >
    > Thanks,
    >
    > M.

    --
    Alex Bennée


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