Re: [PATCH v5 04/19] firmware/psci: Add psci_early_test_conduit()

[Steven Price <steven.price@arm.com>]

On 23/08/2024 14:29, Will Deacon wrote:
> On Mon, Aug 19, 2024 at 02:19:09PM +0100, Steven Price wrote:
>> From: Jean-Philippe Brucker <jean-philippe@linaro.org>
>>
>> Add a function to test early if PSCI is present and what conduit it
>> uses. Because the PSCI conduit corresponds to the SMCCC one, this will
>> let the kernel know whether it can use SMC instructions to discuss with
>> the Realm Management Monitor (RMM), early enough to enable RAM and
>> serial access when running in a Realm.
>>
>> Signed-off-by: Jean-Philippe Brucker <jean-philippe@linaro.org>
>> Signed-off-by: Steven Price <steven.price@arm.com>
>> ---
>> v4: New patch
>> ---
>>  drivers/firmware/psci/psci.c | 25 +++++++++++++++++++++++++
>>  include/linux/psci.h         |  5 +++++
>>  2 files changed, 30 insertions(+)
>>
>> diff --git a/drivers/firmware/psci/psci.c b/drivers/firmware/psci/psci.c
>> index 2328ca58bba6..2b308f97ef2c 100644
>> --- a/drivers/firmware/psci/psci.c
>> +++ b/drivers/firmware/psci/psci.c
>> @@ -13,6 +13,7 @@
>>  #include <linux/errno.h>
>>  #include <linux/linkage.h>
>>  #include <linux/of.h>
>> +#include <linux/of_fdt.h>
>>  #include <linux/pm.h>
>>  #include <linux/printk.h>
>>  #include <linux/psci.h>
>> @@ -769,6 +770,30 @@ int __init psci_dt_init(void)
>>  	return ret;
>>  }
>>  
>> +/*
>> + * Test early if PSCI is supported, and if its conduit matches @conduit
>> + */
>> +bool __init psci_early_test_conduit(enum arm_smccc_conduit conduit)
>> +{
>> +	int len;
>> +	int psci_node;
>> +	const char *method;
>> +	unsigned long dt_root;
>> +
>> +	/* DT hasn't been unflattened yet, we have to work with the flat blob */
>> +	dt_root = of_get_flat_dt_root();
>> +	psci_node = of_get_flat_dt_subnode_by_name(dt_root, "psci");
>> +	if (psci_node <= 0)
>> +		return false;
>> +
>> +	method = of_get_flat_dt_prop(psci_node, "method", &len);
>> +	if (!method)
>> +		return false;
>> +
>> +	return  (conduit == SMCCC_CONDUIT_SMC && strncmp(method, "smc", len) == 0) ||
>> +		(conduit == SMCCC_CONDUIT_HVC && strncmp(method, "hvc", len) == 0);
>> +}
> 
> This still looks incomplete to me as per my earlier comments:
> 
> https://lore.kernel.org/all/20240709104851.GE12978@willie-the-truck/
> 
> For the first implementation, can we punt the RIPAS_RAM to the bootloader
> and drop support for earlycon? Even if we manage to shoe-horn enough code
> into the early boot path, I think we'll regret it later on because there's
> always something that wants to be first and it inevitably ends up being
> a nightmare to maintain.

Short-answer: yes, although it has drawbacks.

I've never been keen on the RIPAS_RAM requirement, the logic behind it
is that it makes it easier to have varying amounts of RAM given to the
guest without affecting the attestation. But it's a weak argument and
I'd personally prefer to punt the responsibility to a bootloader/VMM.

earlycon should be fairly easy to remove - and it doesn't have to
actually kill earlycon because we can pass in the address with the top
bit set - it just requires fixing up the VMM.

EFI is the main issue.

I'll have a go at coming up with a cut down series - at the very least
I'll see if I can rearrange to have the troublesome parts at the end so
they can be dropped if necessary.

Steve