Re: [PATCH v2] x86/kdump: Reserve extra memory when SME or SEV is active

[Borislav Petkov]

On Mon, Aug 26, 2019 at 12:45:35PM +0800, Kairui Song wrote:
> Since commit c7753208a94c ("x86, swiotlb: Add memory encryption support"),
> SWIOTLB will be enabled even if there is less than 4G of memory when SME
> is active, to support DMA of devices that not support address with the
> encrypt bit.
> 
> And commit aba2d9a6385a ("iommu/amd: Do not disable SWIOTLB if SME is
> active") make the kernel keep SWIOTLB enabled even if there is an IOMMU.
> 
> Then commit d7b417fa08d1 ("x86/mm: Add DMA support for SEV memory
> encryption") will always force SWIOTLB to be enabled when SEV is active
> in all cases.
> 
> Now, when either SME or SEV is active, SWIOTLB will be force enabled,
> and this is also true for kdump kernel. As a result kdump kernel will
> run out of already scarce pre-reserved memory easily.
> 
> So when SME/SEV is active, reserve extra memory for SWIOTLB to ensure
> kdump kernel have enough memory, except when "crashkernel=size[KMG],high"
> is specified or any offset is used. As for the high reservation case, an
> extra low memory region will always be reserved and that is enough for
> SWIOTLB. Else if the offset format is used, user should be fully aware
> of any possible kdump kernel memory requirement and have to organize the
> memory usage carefully.
> 
> Signed-off-by: Kairui Song <kasong@xxxxxxxxxx>
> 
> ---
> Update from V1:
> - Use mem_encrypt_active() instead of "sme_active() || sev_active()"
> - Don't reserve extra memory when ",high" or "@offset" is used, and
>   don't print redundant message.
> - Fix coding style problem
> 
>  arch/x86/kernel/setup.c | 31 ++++++++++++++++++++++++++++---
>  1 file changed, 28 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/kernel/setup.c b/arch/x86/kernel/setup.c
> index bbe35bf879f5..221beb10c55d 100644
> --- a/arch/x86/kernel/setup.c
> +++ b/arch/x86/kernel/setup.c
> @@ -528,7 +528,7 @@ static int __init reserve_crashkernel_low(void)
>  
>  static void __init reserve_crashkernel(void)
>  {
> -	unsigned long long crash_size, crash_base, total_mem;
> +	unsigned long long crash_size, crash_base, total_mem, mem_enc_req;
>  	bool high = false;
>  	int ret;
>  
> @@ -550,6 +550,15 @@ static void __init reserve_crashkernel(void)
>  		return;
>  	}
>  
> +	/*
> +	 * When SME/SEV is active, it will always required an extra SWIOTLB
> +	 * region.
> +	 */
> +	if (mem_encrypt_active())
> +		mem_enc_req = ALIGN(swiotlb_size_or_default(), SZ_1M);
> +	else
> +		mem_enc_req = 0;

Hmm, ugly.

You set mem_enc_reg here ...

> +
>  	/* 0 means: find the address automatically */
>  	if (!crash_base) {
>  		/*
> @@ -563,11 +572,19 @@ static void __init reserve_crashkernel(void)
>  		if (!high)
>  			crash_base = memblock_find_in_range(CRASH_ALIGN,
>  						CRASH_ADDR_LOW_MAX,
> -						crash_size, CRASH_ALIGN);
> -		if (!crash_base)
> +						crash_size + mem_enc_req,
> +						CRASH_ALIGN);
> +		/*
> +		 * For high reservation, an extra low memory for SWIOTLB will
> +		 * always be reserved later, so no need to reserve extra
> +		 * memory for memory encryption case here.
> +		 */
> +		if (!crash_base) {
> +			mem_enc_req = 0;

... but you clear it here...

>  			crash_base = memblock_find_in_range(CRASH_ALIGN,
>  						CRASH_ADDR_HIGH_MAX,
>  						crash_size, CRASH_ALIGN);
> +		}
>  		if (!crash_base) {
>  			pr_info("crashkernel reservation failed - No suitable area found.\n");
>  			return;
> @@ -575,6 +592,7 @@ static void __init reserve_crashkernel(void)
>  	} else {
>  		unsigned long long start;
>  
> +		mem_enc_req = 0;

... and here...

>  		start = memblock_find_in_range(crash_base,
>  					       crash_base + crash_size,
>  					       crash_size, 1 << 20);
> @@ -583,6 +601,13 @@ static void __init reserve_crashkernel(void)
>  			return;
>  		}
>  	}
> +
> +	if (mem_enc_req) {
> +		pr_info("Memory encryption is active, crashkernel needs %ldMB extra memory\n",
> +			(unsigned long)(mem_enc_req >> 20));
> +		crash_size += mem_enc_req;
> +	}

... and then you report only when it is still set.

How about you carve out that if (!crash_base) { ... } else { } piece
into a separate function without any further changes - only code
movement? That is your patch 1.

Your patch 2 is then adding the mem_encrypt_active() check in the if
(!crash_base && !high) case, i.e., only where you need it and issuing
the pr_info from there instead of stretching that logic throughout the
whole function and twisting my brain unnecessarily?

Thx.

-- 
Regards/Gruss,
    Boris.

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