linux-kernel - Re: [PATCH 3/3] efi: Add support for using efivars as a pstore backend

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Message-ID: <BANLkTi=qmS090MK=sx=xMN95zD5AtNkuLG_g092oVXJezoRCYA@mail.gmail.com>
Date:	Tue, 21 Jun 2011 13:16:29 -0700
From:	Mike Waychison <mikew@...gle.com>
To:	Matthew Garrett <mjg59@...f.ucam.org>
Cc:	tony.luck@...el.com, linux-kernel@...r.kernel.org,
	Matt_Domsch@...l.com
Subject: Re: [PATCH 3/3] efi: Add support for using efivars as a pstore backend

On Tue, Jun 21, 2011 at 8:10 AM, Matthew Garrett <mjg59@...f.ucam.org> wrote:
> On Mon, Jun 20, 2011 at 05:56:27PM -0700, Mike Waychison wrote:
>> On 06/06/11 12:38, Matthew Garrett wrote:
>> >EFI provides an area of nonvolatile storage managed by the firmware. We
>> >can use this as a pstore backend to maintain copies of oopses, aiding
>> >diagnosis.
>>
>> How do I configure this thing?
>
> You don't. I'll be posting a patch for pstore that lets you choose the
> backend - that can be used to disable this functionality at boot time.

Hmm.  Okay.

>
>> >@@ -387,12 +400,145 @@ static struct kobj_type efivar_ktype = {
>> >     .default_attrs = def_attrs,
>> >  };
>> >
>> >+static struct efivar_entry *walk_entry;
>> >+
>> >+static struct pstore_info efi_pstore_info;
>>
>> Can you move these into struct efivars in efi.h?
>
> In theory, but I don't really understand the benefit. You can't have
> more than one efivars implementation on a system.

It's just cleaner and keeps the state of things in a single place,
rather than in globals.  I just cleaned all this global state up, and
would like to keep it clean.

>
>> >+static u64 efi_pstore_write(enum pstore_type_id type, int part, size_t size,
>> >+                        struct pstore_info *psi)
>> >+{
>> >+    char name[DUMP_NAME_LEN];
>> >+    char stub_name[DUMP_NAME_LEN];
>> >+    efi_char16_t efi_name[DUMP_NAME_LEN];
>> >+    efi_guid_t vendor = LINUX_EFI_CRASH_GUID;
>> >+    struct efivars *efivars = psi->data;
>> >+    struct efivar_entry *entry, *found = NULL;
>> >+    int i;
>> >+
>> >+    sprintf(stub_name, "dump-type%u-%u-", type, part);
>>
>> The format specifier here uses an unsigned, but your series passes
>> part around as a signed int.  If part is truely non-negative,
>> consider changing it to unsigned?
>
> The variable name is fundamentally meaningless. Just think of it as a
> binary representation of the data. Formatting it as a signed integer
> would break the parsing.

What do you mean it would break the parsing?

> But you're right that there's probably no
> reason for it to be signed in the first place - Tony?
>
>> >+    list_for_each_entry(entry,&efivars->list, list) {
>> >+            get_var_data_locked(efivars,&entry->var);
>> >+
>> >+            for (i = 0; i<  DUMP_NAME_LEN; i++) {
>> >+                    if (efi_name[i] == 0) {
>>
>> Test for entry->var.VariableName[i] == 0 too.  Actually, could we
>> just turn this string comparing loop into a strncmp test?
>
> The idea is to check for prefixes. If efi_name[i] is non-zero but
> VariableName[i] is zero then we'll break due to them not matching, which
> is the desired behaviour.

That's fine, but it's not what the code does.  It's also a lot clearer
to the reader if this isn't open coded.  We should also be checking
that this variable is ours with a guid check.  I'd be happier with a
utf16_strncmp here.  Will follow up with patches that stack on top of
yours.

>> >  static ssize_t efivar_create(struct file *filp, struct kobject *kobj,
>> >                          struct bin_attribute *bin_attr,
>> >@@ -763,6 +909,14 @@ int register_efivars(struct efivars *efivars,
>> >     if (error)
>> >             unregister_efivars(efivars);
>> >
>> >+    efi_pstore_info.buf = kmalloc(4096, GFP_KERNEL);
>> >+    if (efi_pstore_info.buf) {
>> >+            efi_pstore_info.bufsize = 1024;
>> >+            efi_pstore_info.data = efivars;
>> >+            mutex_init(&efi_pstore_info.buf_mutex);
>> >+            pstore_register(&efi_pstore_info);
>> >+    }
>> >+
>>
>> Hmm.  pstore doesn't have a pstore_unregister?  This is unfortunate
>> because this breaks efivars module unloading :(
>
> Userspace really ought to depend on efivars being available on EFI
> systems. I don't think losing the ability to unload it is a big loss.

I don't know of any such dependencies.  Am I missing something?

I'll follow up with more patches that apply on top of yours.  I'm not
happy with the string operations in the driver as it is, and I've
cleaned some of this up.  Feel free to collapse whatever is needed
from them into your series or pick them up as is.
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