Re: [PATCH v10 9/9] KVM: Enable and expose KVM_MEM_PRIVATE

[Xiaoyao Li <xiaoyao.li@intel.com>]

On 3/24/2023 10:10 AM, Chao Peng wrote:
> On Wed, Mar 22, 2023 at 05:41:31PM -0700, Isaku Yamahata wrote:
>> On Wed, Mar 08, 2023 at 03:40:26PM +0800,
>> Chao Peng <chao.p.peng@linux.intel.com> wrote:
>>
>>> On Wed, Mar 08, 2023 at 12:13:24AM +0000, Ackerley Tng wrote:
>>>> Chao Peng <chao.p.peng@linux.intel.com> writes:
>>>>
>>>>> On Sat, Jan 14, 2023 at 12:01:01AM +0000, Sean Christopherson wrote:
>>>>>> On Fri, Dec 02, 2022, Chao Peng wrote:
>>>>> ...
>>>>>> Strongly prefer to use similar logic to existing code that detects wraps:
>>>>
>>>>>> 		mem->restricted_offset + mem->memory_size < mem->restricted_offset
>>>>
>>>>>> This is also where I'd like to add the "gfn is aligned to offset"
>>>>>> check, though
>>>>>> my brain is too fried to figure that out right now.
>>>>
>>>>> Used count_trailing_zeros() for this TODO, unsure we have other better
>>>>> approach.
>>>>
>>>>> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
>>>>> index afc8c26fa652..fd34c5f7cd2f 100644
>>>>> --- a/virt/kvm/kvm_main.c
>>>>> +++ b/virt/kvm/kvm_main.c
>>>>> @@ -56,6 +56,7 @@
>>>>>    #include <asm/processor.h>
>>>>>    #include <asm/ioctl.h>
>>>>>    #include <linux/uaccess.h>
>>>>> +#include <linux/count_zeros.h>
>>>>
>>>>>    #include "coalesced_mmio.h"
>>>>>    #include "async_pf.h"
>>>>> @@ -2087,6 +2088,19 @@ static bool kvm_check_memslot_overlap(struct
>>>>> kvm_memslots *slots, int id,
>>>>>    	return false;
>>>>>    }
>>>>
>>>>> +/*
>>>>> + * Return true when ALIGNMENT(offset) >= ALIGNMENT(gpa).
>>>>> + */
>>>>> +static bool kvm_check_rmem_offset_alignment(u64 offset, u64 gpa)
>>>>> +{
>>>>> +	if (!offset)
>>>>> +		return true;
>>>>> +	if (!gpa)
>>>>> +		return false;
>>>>> +
>>>>> +	return !!(count_trailing_zeros(offset) >= count_trailing_zeros(gpa));
>>
>> This check doesn't work expected. For example, offset = 2GB, gpa=4GB
>> this check fails.
> 
> This case is expected to fail as Sean initially suggested[*]:
>    I would rather reject memslot if the gfn has lesser alignment than
>    the offset. I'm totally ok with this approach _if_ there's a use case.
>    Until such a use case presents itself, I would rather be conservative
>    from a uAPI perspective.
> 
> I understand that we put tighter restriction on this but if you see such
> restriction is really a big issue for real usage, instead of a
> theoretical problem, then we can loosen the check here. But at that time
> below code is kind of x86 specific and may need improve.
> 
> BTW, in latest code, I replaced count_trailing_zeros() with fls64():
>    return !!(fls64(offset) >= fls64(gpa));

wouldn't it be !!(ffs64(offset) <= ffs64(gpa)) ?

> [*] https://lore.kernel.org/all/Y8HldeHBrw+OOZVm@google.com/
> 
> Chao
>> I come up with the following.
>>
>> >From ec87e25082f0497431b732702fae82c6a05071bf Mon Sep 17 00:00:00 2001
>> Message-Id: <ec87e25082f0497431b732702fae82c6a05071bf.1679531995.git.isaku.yamahata@intel.com>
>> From: Isaku Yamahata <isaku.yamahata@intel.com>
>> Date: Wed, 22 Mar 2023 15:32:56 -0700
>> Subject: [PATCH] KVM: Relax alignment check for restricted mem
>>
>> kvm_check_rmem_offset_alignment() only checks based on offset alignment
>> and GPA alignment.  However, the actual alignment for offset depends
>> on architecture.  For x86 case, it can be 1G, 2M or 4K.  So even if
>> GPA is aligned for 1G+, only 1G-alignment is required for offset.
>>
>> Without this patch, gpa=4G, offset=2G results in failure of memory slot
>> creation.
>>
>> Fixes: edc8814b2c77 ("KVM: Require gfn be aligned with restricted offset")
>> Signed-off-by: Isaku Yamahata <isaku.yamahata@intel.com>
>> ---
>>   arch/x86/include/asm/kvm_host.h | 15 +++++++++++++++
>>   virt/kvm/kvm_main.c             |  9 ++++++++-
>>   2 files changed, 23 insertions(+), 1 deletion(-)
>>
>> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
>> index 88e11dd3afde..03af44650f24 100644
>> --- a/arch/x86/include/asm/kvm_host.h
>> +++ b/arch/x86/include/asm/kvm_host.h
>> @@ -16,6 +16,7 @@
>>   #include <linux/irq_work.h>
>>   #include <linux/irq.h>
>>   #include <linux/workqueue.h>
>> +#include <linux/count_zeros.h>
>>   
>>   #include <linux/kvm.h>
>>   #include <linux/kvm_para.h>
>> @@ -143,6 +144,20 @@
>>   #define KVM_HPAGE_MASK(x)	(~(KVM_HPAGE_SIZE(x) - 1))
>>   #define KVM_PAGES_PER_HPAGE(x)	(KVM_HPAGE_SIZE(x) / PAGE_SIZE)
>>   
>> +#define kvm_arch_required_alignment	kvm_arch_required_alignment
>> +static inline int kvm_arch_required_alignment(u64 gpa)
>> +{
>> +	int zeros = count_trailing_zeros(gpa);
>> +
>> +	WARN_ON_ONCE(!PAGE_ALIGNED(gpa));
>> +	if (zeros >= KVM_HPAGE_SHIFT(PG_LEVEL_1G))
>> +		return KVM_HPAGE_SHIFT(PG_LEVEL_1G);
>> +	else if (zeros >= KVM_HPAGE_SHIFT(PG_LEVEL_2M))
>> +		return KVM_HPAGE_SHIFT(PG_LEVEL_2M);
>> +
>> +	return PAGE_SHIFT;
>> +}
>> +
>>   #define KVM_MEMSLOT_PAGES_TO_MMU_PAGES_RATIO 50
>>   #define KVM_MIN_ALLOC_MMU_PAGES 64UL
>>   #define KVM_MMU_HASH_SHIFT 12
>> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
>> index c9c4eef457b0..f4ff96171d24 100644
>> --- a/virt/kvm/kvm_main.c
>> +++ b/virt/kvm/kvm_main.c
>> @@ -2113,6 +2113,13 @@ static bool kvm_check_memslot_overlap(struct kvm_memslots *slots, int id,
>>   	return false;
>>   }
>>   
>> +#ifndef kvm_arch_required_alignment
>> +__weak int kvm_arch_required_alignment(u64 gpa)
>> +{
>> +	return PAGE_SHIFT
>> +}
>> +#endif
>> +
>>   /*
>>    * Return true when ALIGNMENT(offset) >= ALIGNMENT(gpa).
>>    */
>> @@ -2123,7 +2130,7 @@ static bool kvm_check_rmem_offset_alignment(u64 offset, u64 gpa)
>>   	if (!gpa)
>>   		return false;
>>   
>> -	return !!(count_trailing_zeros(offset) >= count_trailing_zeros(gpa));
>> +	return !!(count_trailing_zeros(offset) >= kvm_arch_required_alignment(gpa));
>>   }
>>   
>>   /*
>> -- 
>> 2.25.1
>>
>>
>>
>> -- 
>> Isaku Yamahata <isaku.yamahata@gmail.com>