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Date:   Fri, 10 May 2019 10:59:01 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     kvm@...r.kernel.org, virtualization@...ts.linux-foundation.org,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        Christoph Hellwig <hch@...radead.org>,
        James Bottomley <James.Bottomley@...senpartnership.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Darren Hart <dvhart@...radead.org>
Subject: Re: [RFC PATCH V2] vhost: don't use kmap() to log dirty pages


On 2019/5/9 下午9:18, Michael S. Tsirkin wrote:
> On Thu, May 09, 2019 at 08:58:00AM -0400, Jason Wang wrote:
>> Vhost log dirty pages directly to a userspace bitmap through GUP and
>> kmap_atomic() since kernel doesn't have a set_bit_to_user()
>> helper. This will cause issues for the arch that has virtually tagged
>> caches. The way to fix is to keep using userspace virtual
>> address. Fortunately, futex has arch_futex_atomic_op_inuser() which
>> could be used for setting a bit to user.
>>
>> Note:
>> - There're archs (few non popular ones) that don't implement futex
>>    helper, we can't log dirty pages. We can fix them e.g for non
>>    virtually tagged archs implement a kmap fallback on top or simply
>>    disable LOG_ALL features of vhost.
>> - The helper also requires userspace pointer is located at 4-byte
>>    boundary, need to check during dirty log setting
> Why check? Round it down.


Will do this.


>
>> Cc: Christoph Hellwig <hch@...radead.org>
>> Cc: James Bottomley <James.Bottomley@...senPartnership.com>
>> Cc: Andrea Arcangeli <aarcange@...hat.com>
>> Cc: Thomas Gleixner <tglx@...utronix.de>
>> Cc: Ingo Molnar <mingo@...hat.com>
>> Cc: Peter Zijlstra <peterz@...radead.org>
>> Cc: Darren Hart <dvhart@...radead.org>
>> Fixes: 3a4d5c94e9593 ("vhost_net: a kernel-level virtio server")
>> Signed-off-by: Jason Wang <jasowang@...hat.com>
>> ---
>> Changes from V1:
>> - switch to use arch_futex_atomic_op_inuser()
>> ---
>>   drivers/vhost/vhost.c | 35 +++++++++++++++++------------------
>>   1 file changed, 17 insertions(+), 18 deletions(-)
>>
>> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
>> index 351af88..4e5a004 100644
>> --- a/drivers/vhost/vhost.c
>> +++ b/drivers/vhost/vhost.c
>> @@ -31,6 +31,7 @@
>>   #include <linux/sched/signal.h>
>>   #include <linux/interval_tree_generic.h>
>>   #include <linux/nospec.h>
>> +#include <asm/futex.h>
>>   
>>   #include "vhost.h"
>>   
>> @@ -1652,6 +1653,10 @@ long vhost_dev_ioctl(struct vhost_dev *d, unsigned int ioctl, void __user *argp)
>>   			r = -EFAULT;
>>   			break;
>>   		}
>> +		if (p & 0x3) {
>> +			r = -EINVAL;
>> +			break;
>> +		}
>>   		for (i = 0; i < d->nvqs; ++i) {
>>   			struct vhost_virtqueue *vq;
>>   			void __user *base = (void __user *)(unsigned long)p;
> That's an ABI change and might break some userspace. I don't think
> it's necessary: you are changing individual bits anyway.


Right.


>
>> @@ -1692,31 +1697,27 @@ long vhost_dev_ioctl(struct vhost_dev *d, unsigned int ioctl, void __user *argp)
>>   }
>>   EXPORT_SYMBOL_GPL(vhost_dev_ioctl);
>>   
>> -/* TODO: This is really inefficient.  We need something like get_user()
>> - * (instruction directly accesses the data, with an exception table entry
>> - * returning -EFAULT). See Documentation/x86/exception-tables.txt.
>> - */
>> -static int set_bit_to_user(int nr, void __user *addr)
>> +static int set_bit_to_user(int nr, u32 __user *addr)
>>   {
>>   	unsigned long log = (unsigned long)addr;
>>   	struct page *page;
>> -	void *base;
>> -	int bit = nr + (log % PAGE_SIZE) * 8;
>> +	u32 old;
>>   	int r;
>>   
>>   	r = get_user_pages_fast(log, 1, 1, &page);
> OK so the trick is that page is pinned so you don't expect
> arch_futex_atomic_op_inuser below to fail.  get_user_pages_fast
> guarantees page is not going away but does it guarantee PTE won't be
> invaidated or write protected?


Good point, then I think we probably need to do manual fixup through 
fixup_user_fault() if arch_futex_atomic_op_in_user() fail.


>
>>   	if (r < 0)
>>   		return r;
>>   	BUG_ON(r != 1);
>> -	base = kmap_atomic(page);
>> -	set_bit(bit, base);
>> -	kunmap_atomic(base);
>> +
>> +	r = arch_futex_atomic_op_inuser(FUTEX_OP_ADD, 1 << nr, &old, addr);
>> +	/* TODO: fallback to kmap() when -ENOSYS? */
>> +
> Add a comment why this won't fail? Maybe warn on EFAULT?
>
> Also down the road a variant that does not need tricks like this is
> still nice to have.


Ok. Let me post a V3.

Thanks


>
>
>>   	set_page_dirty_lock(page);
>>   	put_page(page);
>> -	return 0;
>> +	return r;
>>   }
>>   
>> -static int log_write(void __user *log_base,
>> +static int log_write(u32 __user *log_base,
>>   		     u64 write_address, u64 write_length)
>>   {
>>   	u64 write_page = write_address / VHOST_PAGE_SIZE;
>> @@ -1726,12 +1727,10 @@ static int log_write(void __user *log_base,
>>   		return 0;
>>   	write_length += write_address % VHOST_PAGE_SIZE;
>>   	for (;;) {
>> -		u64 base = (u64)(unsigned long)log_base;
>> -		u64 log = base + write_page / 8;
>> -		int bit = write_page % 8;
>> -		if ((u64)(unsigned long)log != log)
>> -			return -EFAULT;
>> -		r = set_bit_to_user(bit, (void __user *)(unsigned long)log);
>> +		u32 __user *log = log_base + write_page / 32;
>> +		int bit = write_page % 32;
>> +
>> +		r = set_bit_to_user(bit, log);
>>   		if (r < 0)
>>   			return r;
>>   		if (write_length <= VHOST_PAGE_SIZE)
>> -- 
>> 1.8.3.1

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