Re: [PATCH v5 10/10] arm_mpam: detect and enable MPAM-Fb PCC support

[Lee Trager <lee@trager.us>]

On 7/29/26 6:41 AM, Andre Przywara wrote:

> @@ -2231,7 +2316,7 @@ static void mpam_msc_drv_remove(struct platform_device *pdev)
>   static struct mpam_msc *do_mpam_msc_drv_probe(struct platform_device *pdev)
>   {
>   	int err;
> -	u32 tmp;
> +	u32 pcc_subspace_id;
>   	struct mpam_msc *msc;
>   	struct resource *msc_res;
>   	struct device *dev = &pdev->dev;
> @@ -2279,7 +2364,7 @@ static struct mpam_msc *do_mpam_msc_drv_probe(struct platform_device *pdev)
>   	if (err)
>   		return ERR_PTR(err);
>   
> -	if (device_property_read_u32(&pdev->dev, "pcc-channel", &tmp))
> +	if (device_property_read_u32(dev, "pcc-channel", &pcc_subspace_id))
>   		msc->iface = MPAM_IFACE_MMIO;
>   	else
>   		msc->iface = MPAM_IFACE_PCC;
> @@ -2295,6 +2380,36 @@ static struct mpam_msc *do_mpam_msc_drv_probe(struct platform_device *pdev)
>   		}
>   		msc->mapped_hwpage_sz = msc_res->end - msc_res->start;
>   		msc->mapped_hwpage = io;
> +	} else if (msc->iface == MPAM_IFACE_PCC) {
> +		int ret;
> +
> +		msc->pcc_chan = mpam_pcc_chan_get(dev, pcc_subspace_id);
> +		if (IS_ERR(msc->pcc_chan)) {
> +			pr_err("Failed to request MSC PCC channel\n");
> +			return ERR_CAST(msc->pcc_chan);
> +		}
> +
> +		if (msc->pcc_chan->pcc_chan->shmem_size < MPAM_FB_MAX_MSG_SIZE) {

I think the PCC shared-memory size undercounts the required region. From 
my understanding shmem_size is the length of the complete PCC 
shared-memory region advertised by the PCCT and mapped by the PCC 
driver. MPAM_FB_MAX_MSG_SIZE is the 4-byte protocol header stored in 
pcc_shmem->command + the 16-byte write payload.

However, mpam_fb_build_write_message() places the payload after the 
complete 16 byte struct acpi_pcct_ext_pcc_shared_memory. The largest 
request therefore occupies 32 bytes of shared memory region(16-byte PCC 
header+ 16-byte write payload). Thus a firmware region between 20 and 31 
bytes currently passes the check but overruns when constructing a write 
request. I think this should be

if (msc->pcc_chan->pcc_chan->shmem_size <

     sizeof(struct acpi_pcct_ext_pcc_shared_memory) +
     MPAM_FB_MAX_MSG_SIZE - MPAM_FB_PROT_HEADER_LEN)

>   
> diff --git a/drivers/resctrl/mpam_internal.h b/drivers/resctrl/mpam_internal.h
> index a2193e7df57c..e926b7feb72a 100644
> --- a/drivers/resctrl/mpam_internal.h
> +++ b/drivers/resctrl/mpam_internal.h
> @@ -77,7 +77,6 @@ struct mpam_msc {
>   	/* Not modified after mpam_is_enabled() becomes true */
>   	enum mpam_msc_iface	iface;
>   	struct mpam_pcc_chan	*pcc_chan;
> -	int			mpam_fb_msc_id;	/* in its own name space */
>   	u32			nrdy_usec;
>   	cpumask_t		accessibility;
>   	bool			has_extd_esr;
> @@ -532,6 +531,11 @@ int mpam_fb_send_write_request(struct mpam_msc *msc, u16 reg, u32 value);
>   int mpam_fb_get_protocol_version(struct mpam_msc *msc);
>   
>   #define MPAM_FB_PROT_HEADER_LEN		sizeof(u32)
> +/* The longest message is MPAM_MSC_WRITE, with 4 parameters. */
> +#define MPAM_FB_MAX_MSG_SIZE	(MPAM_FB_PROT_HEADER_LEN + 4 * sizeof(u32))
> +
> +#define MPAM_FB_VERSION_MAJOR_MASK	GENMASK(31, 16)
> +#define MPAM_FB_VERSION_MINOR_MASK	GENMASK(15, 0)
>   
>   /*
>    * MPAM MSCs have the following register layout. See: