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

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Message-ID: <5750eead-44f9-260f-283d-4902b5363faf@arm.com>
Date:   Wed, 1 Mar 2023 11:55:37 +0000
From:   Steven Price <steven.price@....com>
To:     Zhi Wang <zhi.wang.linux@...il.com>
Cc:     kvm@...r.kernel.org, kvmarm@...ts.linux.dev,
        Catalin Marinas <catalin.marinas@....com>,
        Marc Zyngier <maz@...nel.org>, Will Deacon <will@...nel.org>,
        James Morse <james.morse@....com>,
        Oliver Upton <oliver.upton@...ux.dev>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Zenghui Yu <yuzenghui@...wei.com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Joey Gouly <joey.gouly@....com>,
        Alexandru Elisei <alexandru.elisei@....com>,
        Christoffer Dall <christoffer.dall@....com>,
        Fuad Tabba <tabba@...gle.com>, linux-coco@...ts.linux.dev
Subject: Re: [RFC PATCH 08/28] arm64: RME: Keep a spare page delegated to the
 RMM

On 13/02/2023 16:47, Zhi Wang wrote:
> On Fri, 27 Jan 2023 11:29:12 +0000
> Steven Price <steven.price@....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@....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++) {
>