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Re: [Qemu-devel] [PATCH 4/5] backdoor: [softmmu] Add QEMU-side proxy to

From: Lluís Vilanova
Subject: Re: [Qemu-devel] [PATCH 4/5] backdoor: [softmmu] Add QEMU-side proxy to "libbackdoor.a"
Date: Thu, 29 Sep 2011 23:49:43 +0200
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/23.3 (gnu/linux)

Blue Swirl writes:

> 2011/9/29 Lluís Vilanova <address@hidden>:
>> +static uint64_t control_io_read(void *opaque, target_phys_addr_t addr, 
>> unsigned size)
>> +{
>> +    State *s = opaque;
>> +
>> +    uint64_t res = ldq_p(&s->size);
>> +    uint8_t *resb = (uint8_t*)&res;
>> +    return resb[addr % CTRL_BYTES];

> I don't think these lines do what you mean, but I'm also not sure what
> it is supposed to mean.

Pre: only can read on a byte-per-byte basis (as stated in control_ops.impl),
just because the code looks less ugly, and host performance should not be an
issue here.

The device is treated as a circular buffer of length CTRL_BYTES

Reads are only used to get the size of the data channel.

First line should handle guest/host endianess swapping, although I'm not sure if
that's the API I'm supposed to use.

Then return the N'th byte of the uint64_t variable holding the (endianess-aware)

>> +}
>> +
>> +static void control_io_write(void *opaque, target_phys_addr_t addr, 
>> uint64_t data, unsigned size)
>> +{
>> +    State *s = opaque;
>> +
>> +    uint8_t *cmdb = (uint8_t*)&s->cmd;
>> +    cmdb[addr % CTRL_BYTES] = (uint8_t)data;
>> +
>> +    if ((addr + size) % CTRL_BYTES == 0) {
>> +        qemu_backdoor(ldq_p(&s->cmd), s->data_ptr);
>> +    }

> Same here.

Pre: same as during reads.

Accumulates writes into s->cmd to build the command the guest is sending us (in
guest endianess).

When CTRL_BYTES bytes have been written into the device, get the command value
into host endianess and invoke the user-provided backdoor callback.

This assumes that when executing in KVM, the device handling infrastructure will
get a lock and only one CPU will be sending a backdoor command until completion.

I'll add some comments there and prefix all structs and functions with
"backdoor_", as otherwise debugging could get harder if everyone started
avoiding the prefixes.


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 something new, the whole world becomes that much richer."
 -- The Princess of Pure Reason, as told by Norton Juster in The Phantom

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