On 01/11/2011 06:19 PM, Stefan Berger wrote:
Hi!
I am currently doing some long-term testing of a device model using
memory mapped IO (TPM TIS) and am seeing some strange errors when the
suspend occurs in the middle of a read operation in the Linux TPM TIS
device driver where the driver reads the result packet from the mmio
location.
Short background: The TPM response packet is read in 2 chunks. First
the first 10 bytes are read containing the response's header.
Subsequently the rest of the packet is read using knowledge of the
total size of the response packet from the header (bytes 2-5 in big
endian format). The corresponding code reading the data from the
hardware interface is here:
http://lxr.linux.no/#linux+v2.6.37/drivers/char/tpm/tpm_tis.c#L228
The test I am running is setup as follows:
- inside the VM keys are permanently generated by sending commands to
the TPM; packets read from the interface are dumped to the screen
- on the host a script suspends the VM every 6 seconds and resumes it
immediately afterwards (using libvirt)
As it happens, sometimes the VM is suspended in the middle of a read
operation on the TPM TIS interface -- see above code reference. I see
that because I do dump the state of the TPM TIS when suspending and
see that the read offset is pointing to a location somewhere in the
middle of the packet - so the TPM TIS Linux driver is in the above
loop currently reading the data. I am observing two types of results
if this happens:
- either the result read by the Linux TPM TIS driver is ok, so no
problem here
- or the problematic case where the TPM TIS driver reads a packet with
a byte missing and then at the end gets a zero byte from the TPM TIS
interface indicating that it read beyond the available data. If the
suspend happened while reading the first chunk of data (header), the
TPM TIS driver will also complain that the available data for the 2nd
chunk (burst size) is less than what's expected -- it's an off-by-one
error
So, I then modified the TPM TIS device model to decrement the read
offset pointer by '1' in case it was detected that the suspend
happened in the middle of the read operation -- in Qemu I do this in
the post-load 'method'. This then leads to the following types of
results:
- the problematic(!) case where the read packet was ok
- the expected case where the TPM TIS driver reads the packet and ends
up having two same bytes in the result in consecutive array locations;
besides that the TPM TIS driver will in this case complain that it has
left-over data
So my conclusion from the above tests are:
- for some reason the memory read to the MMIO location happens as the
last instruction executed on suspend and again as the very first on
executed on resume. This explains to me that the TPM TIS model
internal pointer into the packet was advanced by '1' (the packet is
read by subsequently reading from the same memory location) and the
above problematic cases make sense
Most likely this is qemu-kvm failing to obey this snippet from
Documentation/kvm/api.txt:
NOTE: For KVM_EXIT_IO, KVM_EXIT_MMIO and KVM_EXIT_OSI, the corresponding
operations are complete (and guest state is consistent) only after
userspace
has re-entered the kernel with KVM_RUN. The kernel side will first finish
incomplete operations and then check for pending signals. Userspace
can re-enter the guest with an unmasked signal pending to complete
pending operations.
However, the code appears to be correct. kvm_run() calls handle_mmio(),
which returns 0. The following bit
if (!r) {
goto again;
}
at the end of kvm_run() makes it enter the kernel again (delivering a
signal to itself in case we want to stop).
- the other instruction in the Linux TPM TIS drivers that for example
advance the buffer location do not execute twice, i.e., size++ in the
buf[size++] = ... in the Linux driver.
What puzzles me is that the read operation may be run twice but others
don't.
Reads have split execution: kvm emulates the mmio instruction, notices
that it cannot satisfy the read request, exits to qemu, then restarts
the instruction. If the last step is omitted due to savevm, then kvm
will exit back to qemu again and your device will see the read duplicated.
If you have insights as why the above may be occurring, please let me
know. A simple solution to work around this may be to introduce a
register holding the index into the result packet where to read the
next byte from (rather than advancing an internal pointer to the next
byte), though this would deviate the driver from the standard
interface the model currently implements.
Most undesirable, I'd like to fix the bug.
Can you sprinkle some printfs() arount kvm_run (in qemu-kvm.c) to verify
this?
Good pattern:
ioctl(KVM_RUN)
-> KVM_EXIT_MMIO
ioctl(KVM_RUN)
-> ENTR
no further KVM_RUNs
or
ioctl(KVM_RUN)
-> something other than KVM_EXIT_MMIO
no further KVM_RUNs
Bad pattern:
ioctl(KVM_RUN)
-> KVM_EXIT_MMIO
no further KVM_RUNs
(in the save portion of your test)
--
error compiling committee.c: too many arguments to function
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