Re: [RFC PATCH v4 19/28] swiotlb: Add warnings for use of bounce buffers with SME

Tom Lendacky
2017-02-17T16:51:45Z

On 2/17/2017 9:59 AM, Konrad Rzeszutek Wilk wrote:
> On Thu, Feb 16, 2017 at 09:46:19AM -0600, Tom Lendacky wrote:
>> Add warnings to let the user know when bounce buffers are being used for
>> DMA when SME is active. Since the bounce buffers are not in encrypted
>> memory, these notifications are to allow the user to determine some
>> appropriate action - if necessary.
>>
>> Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
>> ---
>> arch/x86/include/asm/mem_encrypt.h | 11 +++++++++++
>> include/linux/dma-mapping.h | 11 +++++++++++
>> include/linux/mem_encrypt.h | 6 ++++++
>> lib/swiotlb.c | 3 +++
>> 4 files changed, 31 insertions(+)
>>
>> diff --git a/arch/x86/include/asm/mem_encrypt.h b/arch/x86/include/asm/mem_encrypt.h
>> index 87e816f..5a17f1b 100644
>> --- a/arch/x86/include/asm/mem_encrypt.h
>> +++ b/arch/x86/include/asm/mem_encrypt.h
>> @@ -26,6 +26,11 @@ static inline bool sme_active(void)
>> return (sme_me_mask) ? true : false;
>> }
>>
>> +static inline u64 sme_dma_mask(void)
>> +{
>> + return ((u64)sme_me_mask << 1) - 1;
>> +}
>> +
>> void __init sme_early_encrypt(resource_size_t paddr,
>> unsigned long size);
>> void __init sme_early_decrypt(resource_size_t paddr,
>> @@ -53,6 +58,12 @@ static inline bool sme_active(void)
>> {
>> return false;
>> }
>> +
>> +static inline u64 sme_dma_mask(void)
>> +{
>> + return 0ULL;
>> +}
>> +
>> #endif
>>
>> static inline void __init sme_early_encrypt(resource_size_t paddr,
>> diff --git a/include/linux/dma-mapping.h b/include/linux/dma-mapping.h
>> index 10c5a17..130bef7 100644
>> --- a/include/linux/dma-mapping.h
>> +++ b/include/linux/dma-mapping.h
>> @@ -10,6 +10,7 @@
>> #include <linux/scatterlist.h>
>> #include <linux/kmemcheck.h>
>> #include <linux/bug.h>
>> +#include <linux/mem_encrypt.h>
>>
>> /**
>> * List of possible attributes associated with a DMA mapping. The semantics
>> @@ -557,6 +558,11 @@ static inline int dma_set_mask(struct device *dev, u64 mask)
>>
>> if (!dev->dma_mask || !dma_supported(dev, mask))
>> return -EIO;
>> +
>> + if (sme_active() && (mask < sme_dma_mask()))
>> + dev_warn(dev,
>> + "SME is active, device will require DMA bounce buffers\n");
>
> You can make it one line. But I am wondering if you should use
> printk_ratelimit as this may fill the console up.

I thought the use of dma_set_mask() was mostly a one time probe/setup
thing so I didn't think we would get that many of these messages. If
dma_set_mask() is called much more often that that I can change this
to a printk_ratelimit(). I'll look into it further.

>
>> +
>> *dev->dma_mask = mask;
>> return 0;
>> }
>> @@ -576,6 +582,11 @@ static inline int dma_set_coherent_mask(struct device *dev, u64 mask)
>> {
>> if (!dma_supported(dev, mask))
>> return -EIO;
>> +
>> + if (sme_active() && (mask < sme_dma_mask()))
>> + dev_warn(dev,
>> + "SME is active, device will require DMA bounce buffers\n");
>
> Ditto.
>> +
>> dev->coherent_dma_mask = mask;
>> return 0;
>> }
>> diff --git a/include/linux/mem_encrypt.h b/include/linux/mem_encrypt.h
>> index 14a7b9f..6829ff1 100644
>> --- a/include/linux/mem_encrypt.h
>> +++ b/include/linux/mem_encrypt.h
>> @@ -28,6 +28,12 @@ static inline bool sme_active(void)
>> {
>> return false;
>> }
>> +
>> +static inline u64 sme_dma_mask(void)
>> +{
>> + return 0ULL;
>> +}
>> +
>> #endif
>>
>> #endif /* CONFIG_AMD_MEM_ENCRYPT */
>> diff --git a/lib/swiotlb.c b/lib/swiotlb.c
>> index c463067..aff9353 100644
>> --- a/lib/swiotlb.c
>> +++ b/lib/swiotlb.c
>> @@ -509,6 +509,9 @@ phys_addr_t swiotlb_tbl_map_single(struct device *hwdev,
>> if (no_iotlb_memory)
>> panic("Can not allocate SWIOTLB buffer earlier and can't now provide you with the DMA bounce buffer");
>>
>> + WARN_ONCE(sme_active(),
>> + "SME is active and system is using DMA bounce buffers\n");
>
> How does that help?
>
> As in what can the user do with this?

It's meant just to notify the user about the condition. The user could
then decide to use an alternative device that supports a greater DMA
range (I can probably change it to a dev_warn_once() so that a device
is identified). I would be nice if I could issue this message once per
device that experienced this. I didn't see anything that would do
that, though.

Thanks,
Tom

>> +
>> mask = dma_get_seg_boundary(hwdev);
>>
>> tbl_dma_addr &= mask;
>>