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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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