Re: [PATCH v2 6/9] gpu: nova-core: use KVVec for SBufferIter flush

[Eliot Courtney]

On Fri Mar 20, 2026 at 1:32 PM JST, Alistair Popple wrote:
> On 2026-03-18 at 18:14 +1100, Eliot Courtney <ecourtney@xxxxxxxxxx> wrote...
>> Change flush_into_kvec to return KVVec instead of KVec. KVVec uses
>> vmalloc for large allocations, which is appropriate since RPC reply
>> payloads can be large (>=20 KiB).
>
> Out of curiosity do you know if there is any upper limit on payload size?

IIRC the largest one I saw in openrm was a few hundred KiB.
Theoretically, the largest complete payload you could have in a single
message is ~64 KiB since we don't support continuation records on the
receive path.

>
> And is there any concern about performance of vmalloc() vs. kmalloc() for RPC
> messages?

`KVVec` uses `KVmalloc` which tries `Kmalloc` first. Most of the time,
`Kmalloc` should work. So this shouldn't regress performance really in
the common case, and it's required for the longer RPCs.

>
>> Update GspSequence to use KVVec accordingly.
>> 
>> Signed-off-by: Eliot Courtney <ecourtney@xxxxxxxxxx>
>> ---
>>  drivers/gpu/nova-core/gsp/sequencer.rs | 4 ++--
>>  drivers/gpu/nova-core/sbuffer.rs       | 6 +++---
>>  2 files changed, 5 insertions(+), 5 deletions(-)
>> 
>> diff --git a/drivers/gpu/nova-core/gsp/sequencer.rs b/drivers/gpu/nova-core/gsp/sequencer.rs
>> index 474e4c8021db..c8f587d2d57b 100644
>> --- a/drivers/gpu/nova-core/gsp/sequencer.rs
>> +++ b/drivers/gpu/nova-core/gsp/sequencer.rs
>> @@ -42,7 +42,7 @@ struct GspSequence {
>>      /// Current command index for error reporting.
>>      cmd_index: u32,
>>      /// Command data buffer containing the sequence of commands.
>> -    cmd_data: KVec<u8>,
>> +    cmd_data: KVVec<u8>,
>>  }
>>  
>>  impl MessageFromGsp for GspSequence {
>> @@ -54,7 +54,7 @@ fn read(
>>          msg: &Self::Message,
>>          sbuffer: &mut SBufferIter<array::IntoIter<&[u8], 2>>,
>>      ) -> Result<Self, Self::InitError> {
>> -        let cmd_data = sbuffer.flush_into_kvec(GFP_KERNEL)?;
>> +        let cmd_data = sbuffer.read_to_vec(GFP_KERNEL)?;
>>          Ok(GspSequence {
>>              cmd_index: msg.cmd_index(),
>>              cmd_data,
>> diff --git a/drivers/gpu/nova-core/sbuffer.rs b/drivers/gpu/nova-core/sbuffer.rs
>> index 3a41d224c77a..ae2facdcbdd4 100644
>> --- a/drivers/gpu/nova-core/sbuffer.rs
>> +++ b/drivers/gpu/nova-core/sbuffer.rs
>> @@ -162,11 +162,11 @@ pub(crate) fn read_exact(&mut self, mut dst: &mut [u8]) -> Result {
>>          Ok(())
>>      }
>>  
>> -    /// Read all the remaining data into a [`KVec`].
>> +    /// Read all the remaining data into a [`KVVec`].
>>      ///
>>      /// `self` will be empty after this operation.
>> -    pub(crate) fn flush_into_kvec(&mut self, flags: kernel::alloc::Flags) -> Result<KVec<u8>> {
>> -        let mut buf = KVec::<u8>::new();
>> +    pub(crate) fn read_to_vec(&mut self, flags: kernel::alloc::Flags) -> Result<KVVec<u8>> {
>> +        let mut buf = KVVec::<u8>::new();
>>  
>>          if let Some(slice) = core::mem::take(&mut self.cur_slice) {
>>              buf.extend_from_slice(slice, flags)?;
>> 
>> -- 
>> 2.53.0
>>