Re: [Qemu-devel] [RFC PATCH 1/4] qemu-char: Convert MemCharDriver to circular buffer

[Lei Li <lilei@linux.vnet.ibm.com>]

On 08/02/2012 05:30 AM, Anthony Liguori wrote:
> Lei Li <lilei@linux.vnet.ibm.com> writes:
>
>> Signed-off-by: Lei Li <lilei@linux.vnet.ibm.com>
>> ---
>>   qemu-char.c |   96 +++++++++++++++++++++++++++++++++++++++++++++++-----------
>>   qemu-char.h |    2 +-
>>   2 files changed, 78 insertions(+), 20 deletions(-)
>>
>> diff --git a/qemu-char.c b/qemu-char.c
>> index c2aaaee..087c92d 100644
>> --- a/qemu-char.c
>> +++ b/qemu-char.c
>> @@ -2517,38 +2517,96 @@ static CharDriverState *qemu_chr_open_socket(QemuOpts *opts)
>>   /***********************************************************/
>>   /* Memory chardev */
>>   typedef struct {
>> -    size_t outbuf_size;
>> -    size_t outbuf_capacity;
>> -    uint8_t *outbuf;
>> +    size_t cbuf_capacity;
>> +    size_t cbuf_in;
>> +    size_t cbuf_out;
>> +    size_t cbuf_count;
>> +    uint8_t *cbuf;
>>   } MemoryDriver;
> Probably should move the buffer into a separate structure and then you
> can drop the cbuf_ prefixes.
>
>> +static int mem_chr_is_empty(CharDriverState *chr)
>> +{
>> +    MemoryDriver *d = chr->opaque;
>> +
>> +    return d->cbuf_count == 0;
>> +}
>> +
>> +static int mem_chr_is_full(CharDriverState *chr)
>> +{
>> +    MemoryDriver *d = chr->opaque;
>> +
>> +    return d->cbuf_count == d->cbuf_capacity;
>> +}
>> +
> Typically, you would use a producer and a consumer index.  To test for
> empty, you would check if (consumer == producer).
>
> To check for full, you would check if ((producer - consumer) == size).
>
> To get the actual index, you always modulus the indexes with size.  This
> only works if size is a power of 2 but that's a reasonable restriction.
>
>>   static int mem_chr_write(CharDriverState *chr, const uint8_t *buf, int len)
>>   {
>>       MemoryDriver *d = chr->opaque;
>> +    int left;
>>   
>> -    /* TODO: the QString implementation has the same code, we should
>> -     * introduce a generic way to do this in cutils.c */
>> -    if (d->outbuf_capacity < d->outbuf_size + len) {
>> -        /* grow outbuf */
>> -        d->outbuf_capacity += len;
>> -        d->outbuf_capacity *= 2;
>> -        d->outbuf = g_realloc(d->outbuf, d->outbuf_capacity);
>> +    if (d->cbuf_capacity < len) {
>> +        return -1;
>>       }
>>   
>> -    memcpy(d->outbuf + d->outbuf_size, buf, len);
>> -    d->outbuf_size += len;
>> +    left = d->cbuf_capacity - d->cbuf_count % d->cbuf_capacity;
>> +
>> +    /* Some of cbuf need to be overwrited */
>> +    if (left < len) {
>> +        memcpy(d->cbuf + d->cbuf_in, buf, left);
>> +        memcpy(d->cbuf + d->cbuf_out, buf + left, len - left);
>> +        d->cbuf_out = (d->cbuf_out + len - left) % d->cbuf_capacity;
>> +        d->cbuf_count = d->cbuf_count + left;
> Doing a mempcy() like this may work, but seems inefficient to me.  I
> think reading like a ring queue works a bit nicer.

Hi Anthony,

What do you mean "reading like a ring queue"?  I am a little confused
here. Could you please give more details?

And thanks for your suggestions. :)

>> +    } else {
>> +        /* Completely overwrite */
>> +        if (mem_chr_is_full(chr)) {
>> +            d->cbuf_out = (d->cbuf_out + len) % d->cbuf_capacity;
>> +        } else {
>> +        /* Enough cbuf to write */
>> +        memcpy(d->cbuf + d->cbuf_in, buf, len);
>> +        d->cbuf_count += len;
>> +        }
> Looks like indenting is off here.
>
> Regards,
>
> Anthony Liguori
>
>> +    }
>> +
>> +    d->cbuf_in = (d->cbuf_in + len) % d->cbuf_capacity;
>>   
>>       return len;
>>   }
>>   
>> -void qemu_chr_init_mem(CharDriverState *chr)
>> +static void mem_chr_read(CharDriverState *chr, uint8_t *buf, int len)
>> +{
>> +    MemoryDriver *d = chr->opaque;
>> +    int left;
>> +
>> +    if (mem_chr_is_empty(chr)) {
>> +        return;
>> +    }
>> +
>> +    left = d->cbuf_capacity - d->cbuf_count % d->cbuf_capacity;
>> +
>> +    if (d->cbuf_capacity < len) {
>> +        len = d->cbuf_capacity;
>> +    }
>> +
>> +    if (left < len) {
>> +        memcpy(buf, d->cbuf + d->cbuf_out, left);
>> +        memcpy(buf + left, d->cbuf + d->cbuf_out + left, len - left);
>> +    } else {
>> +        memcpy(buf, d->cbuf + d->cbuf_out, len);
>> +    }
>> +
>> +    d->cbuf_out = (d->cbuf_out + len) % d->cbuf_capacity;
>> +    d->cbuf_count -= len;
>> +}
>> +
>> +void qemu_chr_init_mem(CharDriverState *chr, size_t size)
>>   {
>>       MemoryDriver *d;
>>   
>>       d = g_malloc(sizeof(*d));
>> -    d->outbuf_size = 0;
>> -    d->outbuf_capacity = 4096;
>> -    d->outbuf = g_malloc0(d->outbuf_capacity);
>> +    d->cbuf_capacity = size;
>> +    d->cbuf_in = 0;
>> +    d->cbuf_out = 0;
>> +    d->cbuf_count = 0;
>> +    d->cbuf = g_malloc0(d->cbuf_capacity);
>>   
>>       memset(chr, 0, sizeof(*chr));
>>       chr->opaque = d;
>> @@ -2558,7 +2616,7 @@ void qemu_chr_init_mem(CharDriverState *chr)
>>   QString *qemu_chr_mem_to_qs(CharDriverState *chr)
>>   {
>>       MemoryDriver *d = chr->opaque;
>> -    return qstring_from_substr((char *) d->outbuf, 0, d->outbuf_size - 1);
>> +    return qstring_from_substr((char *) d->cbuf, 0, d->cbuf_count - 1);
>>   }
>>   
>>   /* NOTE: this driver can not be closed with qemu_chr_delete()! */
>> @@ -2566,7 +2624,7 @@ void qemu_chr_close_mem(CharDriverState *chr)
>>   {
>>       MemoryDriver *d = chr->opaque;
>>   
>> -    g_free(d->outbuf);
>> +    g_free(d->cbuf);
>>       g_free(chr->opaque);
>>       chr->opaque = NULL;
>>       chr->chr_write = NULL;
>> @@ -2575,7 +2633,7 @@ void qemu_chr_close_mem(CharDriverState *chr)
>>   size_t qemu_chr_mem_osize(const CharDriverState *chr)
>>   {
>>       const MemoryDriver *d = chr->opaque;
>> -    return d->outbuf_size;
>> +    return d->cbuf_count;
>>   }
>>   
>>   QemuOpts *qemu_chr_parse_compat(const char *label, const char *filename)
>> diff --git a/qemu-char.h b/qemu-char.h
>> index 486644b..d8d90cc 100644
>> --- a/qemu-char.h
>> +++ b/qemu-char.h
>> @@ -243,7 +243,7 @@ CharDriverState *qemu_chr_open_eventfd(int eventfd);
>>   extern int term_escape_char;
>>   
>>   /* memory chardev */
>> -void qemu_chr_init_mem(CharDriverState *chr);
>> +void qemu_chr_init_mem(CharDriverState *chr, size_t size);
>>   void qemu_chr_close_mem(CharDriverState *chr);
>>   QString *qemu_chr_mem_to_qs(CharDriverState *chr);
>>   size_t qemu_chr_mem_osize(const CharDriverState *chr);
>> -- 
>> 1.7.7.6


-- 
Lei