Re: [RFC PATCH 08/28] arm64: RME: Keep a spare page delegated to the RMM

Steven Price
2023-03-01T11:56:03Z

On 13/02/2023 16:47, Zhi Wang wrote:
> On Fri, 27 Jan 2023 11:29:12 +0000
> Steven Price <steven.price@arm.com> wrote:
>
>> Pages can only be populated/destroyed on the RMM at the 4KB granule,
>> this requires creating the full depth of RTTs. However if the pages are
>> going to be combined into a 4MB huge page the last RTT is only
>> temporarily needed. Similarly when freeing memory the huge page must be
>> temporarily split requiring temporary usage of the full depth oF RTTs.
>>
>> To avoid needing to perform a temporary allocation and delegation of a
>> page for this purpose we keep a spare delegated page around. In
>> particular this avoids the need for memory allocation while destroying
>> the realm guest.
>>
>> Signed-off-by: Steven Price <steven.price@arm.com>
>> ---
>> arch/arm64/include/asm/kvm_rme.h | 3 +++
>> arch/arm64/kvm/rme.c | 6 ++++++
>> 2 files changed, 9 insertions(+)
>>
>> diff --git a/arch/arm64/include/asm/kvm_rme.h b/arch/arm64/include/asm/kvm_rme.h
>> index 055a22accc08..a6318af3ed11 100644
>> --- a/arch/arm64/include/asm/kvm_rme.h
>> +++ b/arch/arm64/include/asm/kvm_rme.h
>> @@ -21,6 +21,9 @@ struct realm {
>> void *rd;
>> struct realm_params *params;
>>
>> + /* A spare already delegated page */
>> + phys_addr_t spare_page;
>> +
>> unsigned long num_aux;
>> unsigned int vmid;
>> unsigned int ia_bits;
>> diff --git a/arch/arm64/kvm/rme.c b/arch/arm64/kvm/rme.c
>> index 9f8c5a91b8fc..0c9d70e4d9e6 100644
>> --- a/arch/arm64/kvm/rme.c
>> +++ b/arch/arm64/kvm/rme.c
>> @@ -148,6 +148,7 @@ static int realm_create_rd(struct kvm *kvm)
>> }
>>
>> realm->rd = rd;
>> + realm->spare_page = PHYS_ADDR_MAX;
>> realm->ia_bits = VTCR_EL2_IPA(kvm->arch.vtcr);
>>
>> if (WARN_ON(rmi_rec_aux_count(rd_phys, &realm->num_aux))) {
>> @@ -357,6 +358,11 @@ void kvm_destroy_realm(struct kvm *kvm)
>> free_page((unsigned long)realm->rd);
>> realm->rd = NULL;
>> }
>> + if (realm->spare_page != PHYS_ADDR_MAX) {
>> + if (!WARN_ON(rmi_granule_undelegate(realm->spare_page)))
>> + free_page((unsigned long)phys_to_virt(realm->spare_page));
>
> Will the page be leaked (not usable for host and realms) if the undelegate
> failed? If yes, better at least put a comment.

Yes - I'll add a comment.

In general being unable to undelegate a page points to a programming
error in the host. The only reason the RMM should refuse the request is
it the page is in use by a Realm which the host has configured. So the
WARN() is correct (there's a kernel bug) and the only sensible course of
action is to leak the page and limp on.

Thanks,

Steve

>> + realm->spare_page = PHYS_ADDR_MAX;
>> + }
>>
>> pgd_sz = kvm_pgd_pages(pgt->ia_bits, pgt->start_level);
>> for (i = 0; i < pgd_sz; i++) {
>