[vbox-dev] New virtualbox vboxvideo mainline kernel broken in Ubuntu

[Michael Thayer]

Hello,

12.10.2017 14:44, Hans de Goede wrote:
[Discussion of getting at guest log data inside a virtual machine.]
> Adding this code from the vboxguest driver I'm upstreaming:
> 
> To some .h file:
> 
> /**
>  * vboxguest logging functions, these log both to the backdoor and call
>  * the equivalent kernel pr_foo function.
>  */
> __printf(1, 2) void vbg_info(const char *fmt, ...);
> __printf(1, 2) void vbg_warn(const char *fmt, ...);
> __printf(1, 2) void vbg_err(const char *fmt, ...);
> 
> To a .c file:
> 
> #define VBG_LOG(name, pr_func) \
> void name(const char *fmt, ...)                                         \
> {                                                                       \
>         unsigned long flags;                                            \
>         va_list args;                                                   \
>         int i, count;                                                   \
>                                                                         \
>         va_start(args, fmt);                                            \
>         spin_lock_irqsave(&vbg_log_lock, flags);                        \
>                                                                         \
>         count = vscnprintf(vbg_log_buf, sizeof(vbg_log_buf), fmt, args);\
>         for (i = 0; i < count; i++)                                     \
>                 outb(vbg_log_buf[i], VBG_DEBUG_PORT);                   \
>                                                                         \
>         pr_func("%s", vbg_log_buf);                                     \
>                                                                         \
>         spin_unlock_irqrestore(&vbg_log_lock, flags);                   \
>         va_end(args);                                                   \
> }                                                                       \
> EXPORT_SYMBOL(name)
> 
> VBG_LOG(vbg_info, pr_info);
> VBG_LOG(vbg_warn, pr_warn);
> VBG_LOG(vbg_err, pr_err);
> 
> And then calling vbg_info / whatever to log should do the trick.
[...]Actually there is another handy trick there (can't remember now if
it works in non-debug builds of VirtualBox) - the machine debugger,
which can detect the dmesg ring buffer in Linux guests without
address-space randomisation, which often means that you can take a look
at it on a hung guest.

Regards
Michael
-- 
Michael Thayer | VirtualBox engineer
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