Re: [kvm-unit-tests PATCH v3 15/27] arm/arm64: mmu_disable: Clean and invalidate before disabling

Nikos Nikoleris
2022-06-30T11:08:52Z

Hi Alex,

On 30/06/2022 11:20, Alexandru Elisei wrote:
> Hi,
>
> On Thu, Jun 30, 2022 at 11:03:12AM +0100, Nikos Nikoleris wrote:
>> From: Andrew Jones <drjones@redhat.com>
>>
>> The commit message of commit 410b3bf09e76 ("arm/arm64: Perform dcache
>> clean + invalidate after turning MMU off") justifies cleaning and
>> invalidating the dcache after disabling the MMU by saying it's nice
>> not to rely on the current page tables and that it should still work
>> (per the spec), as long as there's an identity map in the current
>> tables. Doing the invalidation after also somewhat helped with
>> reenabling the MMU without seeing stale data, but the real problem
>> with reenabling was because the cache needs to be disabled with
>> the MMU, but it wasn't.
>>
>> Since we have to trust/validate that the current page tables have an
>> identity map anyway, then there's no harm in doing the clean
>> and invalidate first (it feels a little better to do so, anyway,
>> considering the cache maintenance instructions take virtual
>> addresses). Then, also disable the cache with the MMU to avoid
>> problems when reenabling. We invalidate the Icache and disable
>> that too for good measure. And, a final TLB invalidation ensures
>> we're crystal clean when we return from asm_mmu_disable().
>
> I'll point you to my previous reply [1] to this exact patch which explains
> why it's incorrect and is only papering over another problem.
>
> [1] https://lore.kernel.org/all/Yn5Z6Kyj62cUNgRN@monolith.localdoman/
>

Apologies, I didn't mean to ignore your feedback on this. There was a
parallel discussion in [2] which I thought makes the problem more concrete.

This is Drew's patch as soon as he confirms he's also happy with the
change you suggested in the patch description I am happy to make it.

Generally, a test will start off with the MMU enabled. At this point, we
access code, use and modify data (EfiLoaderData, EfiLoaderCode). Any of
the two regions could be mapped as any type of memory (I need to have
another look to confirm if it's Normal Memory). Then we want to take
over control of the page tables and for that reason we have to switch
off the MMU. And any access to code or data will be with Device-nGnRnE
as you pointed out. If we don't clean and invalidate, instructions and
data might be in the cache and we will be mixing memory attributes,
won't we?

[2]:
https://lore.kernel.org/all/6c5a3ef7-3742-c4e9-5a94-c702a5b3ebca@arm.com/

Thanks,

Nikos

> Thanks,
> Alex
>
>>
>> Cc: Alexandru Elisei <alexandru.elisei@arm.com>
>> Signed-off-by: Andrew Jones <drjones@redhat.com>
>> Signed-off-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
>> ---
>> arm/cstart.S | 28 +++++++++++++++++++++-------
>> arm/cstart64.S | 21 ++++++++++++++++-----
>> 2 files changed, 37 insertions(+), 12 deletions(-)
>>
>> diff --git a/arm/cstart.S b/arm/cstart.S
>> index 7036e67..dc324c5 100644
>> --- a/arm/cstart.S
>> +++ b/arm/cstart.S
>> @@ -179,6 +179,7 @@ halt:
>> .globl asm_mmu_enable
>> asm_mmu_enable:
>> /* TLBIALL */
>> + mov r2, #0
>> mcr p15, 0, r2, c8, c7, 0
>> dsb nsh
>>
>> @@ -211,12 +212,7 @@ asm_mmu_enable:
>>
>> .globl asm_mmu_disable
>> asm_mmu_disable:
>> - /* SCTLR */
>> - mrc p15, 0, r0, c1, c0, 0
>> - bic r0, #CR_M
>> - mcr p15, 0, r0, c1, c0, 0
>> - isb
>> -
>> + /* Clean + invalidate the entire memory */
>> ldr r0, =__phys_offset
>> ldr r0, [r0]
>> ldr r1, =__phys_end
>> @@ -224,7 +220,25 @@ asm_mmu_disable:
>> sub r1, r1, r0
>> dcache_by_line_op dccimvac, sy, r0, r1, r2, r3
>>
>> - mov pc, lr
>> + /* Invalidate Icache */
>> + mov r0, #0
>> + mcr p15, 0, r0, c7, c5, 0
>> + isb
>> +
>> + /* Disable cache, Icache and MMU */
>> + mrc p15, 0, r0, c1, c0, 0
>> + bic r0, #CR_C
>> + bic r0, #CR_I
>> + bic r0, #CR_M
>> + mcr p15, 0, r0, c1, c0, 0
>> + isb
>> +
>> + /* Invalidate TLB */
>> + mov r0, #0
>> + mcr p15, 0, r0, c8, c7, 0
>> + dsb nsh
>> +
>> + mov pc, lr
>>
>> /*
>> * Vectors
>> diff --git a/arm/cstart64.S b/arm/cstart64.S
>> index e4ab7d0..390feb9 100644
>> --- a/arm/cstart64.S
>> +++ b/arm/cstart64.S
>> @@ -246,11 +246,6 @@ asm_mmu_enable:
>>
>> .globl asm_mmu_disable
>> asm_mmu_disable:
>> - mrs x0, sctlr_el1
>> - bic x0, x0, SCTLR_EL1_M
>> - msr sctlr_el1, x0
>> - isb
>> -
>> /* Clean + invalidate the entire memory */
>> adrp x0, __phys_offset
>> ldr x0, [x0, :lo12:__phys_offset]
>> @@ -259,6 +254,22 @@ asm_mmu_disable:
>> sub x1, x1, x0
>> dcache_by_line_op civac, sy, x0, x1, x2, x3
>>
>> + /* Invalidate Icache */
>> + ic iallu
>> + isb
>> +
>> + /* Disable cache, Icache and MMU */
>> + mrs x0, sctlr_el1
>> + bic x0, x0, SCTLR_EL1_C
>> + bic x0, x0, SCTLR_EL1_I
>> + bic x0, x0, SCTLR_EL1_M
>> + msr sctlr_el1, x0
>> + isb
>> +
>> + /* Invalidate TLB */
>> + tlbi vmalle1
>> + dsb nsh
>> +
>> ret
>>
>> /*
>> --
>> 2.25.1
>>