From: Paolo Bonzini <pbonzini@redhat.com>
To: Peter Maydell <peter.maydell@linaro.org>
Cc: QEMU Developers <qemu-devel@nongnu.org>
Subject: Re: [Qemu-devel] pl031 time across vm save/reload
Date: Fri, 5 Jul 2019 12:13:23 +0200 [thread overview]
Message-ID: <faeb493b-f61b-16fe-3aff-aa40dffaf06b@redhat.com> (raw)
In-Reply-To: <CAFEAcA-giDCA8ZTR3Ft3ZNQUBEJBaXNaMnZOmgPo3Auw1gvZjg@mail.gmail.com>
On 05/07/19 11:58, Peter Maydell wrote:
> On Fri, 5 Jul 2019 at 10:48, Paolo Bonzini <pbonzini@redhat.com> wrote:
>> You're right, the compatibility causes wrong behavior for the default
>> -rtc settings (the RC pauses across migration). The right thing to do
>> would be to store the base rather than the offset: that is, you store
>> the time at which LR was written. Then the offset is s->lr - s->base
>> and it's independent of the machine on which the rtc_clock is being read.
>
> Right. How do we handle this for back-compat purposes? I guess
> we need to have a new migration subsection, so if it's present
> it has the 'base' value and we ignore the 'offset' in the
> main migration data, and if it's not present we assume an
> old->new migration and use the existing offset code. New->old
> migration would not be possible as the new subsection is
> always-present.
Yes, something like that but I would just bump the version. Version 1
has the old meaning for the first field, version 2 has the new meaning.
And also, since our brains are fresh on pl031... currently s->lr is
always 0; besides the bug that writing RTC_LR should update it, the
datasheet says the counter counts up from 1 so perhaps at startup s->lr
should be set to a nonzero value? That would be
qemu_ref_timedate(QEMU_CLOCK_VIRTUAL) - 1.
Paolo
next prev parent reply other threads:[~2019-07-05 10:14 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-07-04 16:02 [Qemu-devel] pl031 time across vm save/reload Peter Maydell
2019-07-05 9:48 ` Paolo Bonzini
2019-07-05 9:58 ` Peter Maydell
2019-07-05 10:13 ` Paolo Bonzini [this message]
2019-07-05 10:21 ` Peter Maydell
2019-07-05 10:32 ` Paolo Bonzini
2019-07-05 10:42 ` Peter Maydell
2019-07-08 17:41 ` Dr. David Alan Gilbert
2019-07-05 10:26 ` Peter Maydell
2019-07-08 14:03 ` Peter Maydell
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