Re: [PATCH] i386/cpu: Expand MAX_FIXED_COUNTERS from 3 to 4 to for Icelake

[Paolo Bonzini]

On 17/03/20 06:54, Like Xu wrote:
> In the Intel SDM, "Table 18-2. Association of Fixed-Function
> Performance Counters with Architectural Performance Events",
> we may have a new fixed counter 'TOPDOWN.SLOTS' (since Icelake),
> which counts the number of available slots for an unhalted
> logical processor. Check commit 6017608936 in the kernel tree.
> 
> Signed-off-by: Like Xu <address@hidden>
> ---
>  target/i386/cpu.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/target/i386/cpu.h b/target/i386/cpu.h
> index 576f309bbf..ec2b67d425 100644
> --- a/target/i386/cpu.h
> +++ b/target/i386/cpu.h
> @@ -1185,7 +1185,7 @@ typedef struct {
>  #define CPU_NB_REGS CPU_NB_REGS32
>  #endif
>  
> -#define MAX_FIXED_COUNTERS 3
> +#define MAX_FIXED_COUNTERS 4
>  #define MAX_GP_COUNTERS    (MSR_IA32_PERF_STATUS - MSR_P6_EVNTSEL0)
>  
>  #define TARGET_INSN_START_EXTRA_WORDS 1
> 

Hi Like, the problem with this patch is that it breaks live migration;
the vmstate_msr_architectural_pmu record hardcodes MAX_FIXED_COUNTERS as
the number of registers.

So it's more complicated, you need to add a new subsection (following
vmstate_msr_architectural_pmu) and transmit it only if the 4th counter
is nonzero (instead of the more complicated check in pmu_enable_needed).
 Just to be safe, I'd make the new subsection hold 16 counters and bump
MAX_FIXED_COUNTERS to 16.

Thanks,

Paolo