Re: [Xen-devel] [PATCH v9 12/15] microcode: reduce memory allocation and copy when creating a patch

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Mon, Aug 26, 2019 at 03:03:22PM +0800, Chao Gao wrote:
> On Fri, Aug 23, 2019 at 10:11:21AM +0200, Roger Pau Monné wrote:
> >On Mon, Aug 19, 2019 at 09:25:25AM +0800, Chao Gao wrote:
> >> To create a microcode patch from a vendor-specific update,
> >> allocate_microcode_patch() copied everything from the update.
> >> It is not efficient. Essentially, we just need to go through
> >> ucodes in the blob, find the one with the newest revision and
> >> install it into the microcode_patch. In the process, buffers
> >> like mc_amd, equiv_cpu_table (on AMD side), and mc (on Intel
> >> side) can be reused. microcode_patch now is allocated after
> >> it is sure that there is a matching ucode.
> >
> >Oh, I think this answers my question on a previous patch.
> >
> >For future series it would be nice to avoid so many rewrites in the
> >same series, alloc_microcode_patch is already modified in a previous
> >patch, just to be removed here. It also makes it harder to follow
> >what's going on.
> 
> Got it. This patch is added in this new version. And some trivial
> patches already got reviewed-by. So I don't merge it with them.
> 
> >>      while ( (error = get_ucode_from_buffer_amd(mc_amd, buf, bufsize,
> >>                                                 &offset)) == 0 )
> >>      {
> >> -        struct microcode_patch *new_patch = alloc_microcode_patch(mc_amd);
> >> -
> >> -        if ( IS_ERR(new_patch) )
> >> -        {
> >> -            error = PTR_ERR(new_patch);
> >> -            break;
> >> -        }
> >> -
> >>          /*
> >> -         * If the new patch covers current CPU, compare patches and store 
> >> the
> >> +         * If the new ucode covers current CPU, compare ucodes and store 
> >> the
> >>           * one with higher revision.
> >>           */
> >> -        if ( (microcode_fits(new_patch->mc_amd) != MIS_UCODE) &&
> >> -             (!patch || (compare_patch(new_patch, patch) == NEW_UCODE)) )
> >> +#define REV_ID(mpb) (((struct microcode_header_amd 
> >> *)(mpb))->processor_rev_id)
> >> +        if ( (microcode_fits(mc_amd) != MIS_UCODE) &&
> >> +             (!saved || (REV_ID(mc_amd->mpb) > REV_ID(saved))) )
> >> +#undef REV_ID
> >>          {
> >> -            struct microcode_patch *tmp = patch;
> >> -
> >> -            patch = new_patch;
> >> -            new_patch = tmp;
> >> +            xfree(saved);
> >> +            saved = mc_amd->mpb;
> >> +            saved_size = mc_amd->mpb_size;
> >>          }
> >> -
> >> -        if ( new_patch )
> >> -            microcode_free_patch(new_patch);
> >> +        else
> >> +            xfree(mc_amd->mpb);
> 
> It might be better to move 'mc_amd->mpb = NULL' here.
> 
> >>  
> >>          if ( offset >= bufsize )
> >>              break;
> >> @@ -593,9 +548,25 @@ static struct microcode_patch 
> >> *cpu_request_microcode(const void *buf,
> >>               *(const uint32_t *)(buf + offset) == UCODE_MAGIC )
> >>              break;
> >>      }
> >> -    xfree(mc_amd->mpb);
> >> -    xfree(mc_amd->equiv_cpu_table);
> >> -    xfree(mc_amd);
> >> +
> >> +    if ( saved )
> >> +    {
> >> +        mc_amd->mpb = saved;
> >> +        mc_amd->mpb_size = saved_size;
> >> +        patch = xmalloc(struct microcode_patch);
> >> +        if ( patch )
> >> +            patch->mc_amd = mc_amd;
> >> +        else
> >> +        {
> >> +            free_patch(mc_amd);
> >> +            error = -ENOMEM;
> >> +        }
> >> +    }
> >> +    else
> >> +    {
> >> +        mc_amd->mpb = NULL;
> >
> >What's the point in setting mpb to NULL if you are just going to free
> >mc_amd below?
> 
> To avoid double free here. mc_amd->mpb is always freed or saved.
> And here we want to free mc_amd itself and mc_amd->equiv_cpu_table.

But there's no chance of a double free here, since you are freeing
mc_amd in the line below after setting mpb = NULL.

I think it would make sense to set mpb = NULL after freeing it inside
the loop.

With that you can add my:

Reviewed-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>

> >
> >Also, I'm not sure I understand why you need to free mc_amd, isn't
> >this buff memory that should be freed by the caller?
> 
> But mc_amd is allocated in this function.
> 
> >
> >ie: in the Intel counterpart below you don't seem to free the mc
> >cursor used for the get_next_ucode_from_buffer loop.
> 
> 'mc' is saved if it is newer than current patch stored in 'saved'.
> Otherwise 'mc' is freed immediately. So we don't need to free it
> again after the while loop.

Ack, thanks!

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