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@xxxxxxxxxx>
>>
>> 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@xxxxxxxxxx>
>> Signed-off-by: Steven Price <steven.price@xxxxxxx>
>> ---
>> 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
