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Date: Mon, 6 May 2019 22:23:29 -0400 From: Jason Wang <jasowang@...hat.com> To: mst@...hat.com, jasowang@...hat.com, kvm@...r.kernel.org, virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org, linux-kernel@...r.kernel.org Cc: Christoph Hellwig <hch@...radead.org>, James Bottomley <James.Bottomley@...senPartnership.com>, Andrea Arcangeli <aarcange@...hat.com> Subject: [PATCH RFC] vhost: don't use kmap() to log dirty pages 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 a cmpxchg to userspace memory helper futex_atomic_cmpxchg_inatomic(). So switch to use it to exchange the userspace bitmap with zero, set the bit and then write it back through put_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 on top or simply disable LOG_ALL features of vhost. Cc: Christoph Hellwig <hch@...radead.org> Cc: James Bottomley <James.Bottomley@...senPartnership.com> Cc: Andrea Arcangeli <aarcange@...hat.com> Fixes: 3a4d5c94e9593 ("vhost_net: a kernel-level virtio server") Signed-off-by: Jason Wang <jasowang@...hat.com> --- drivers/vhost/vhost.c | 27 +++++++++++++++------------ 1 file changed, 15 insertions(+), 12 deletions(-) diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c index 351af88..9c94c41 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" @@ -1692,25 +1693,27 @@ 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_log; int r; r = get_user_pages_fast(log, 1, 1, &page); if (r < 0) return r; BUG_ON(r != 1); - base = kmap_atomic(page); - set_bit(bit, base); - kunmap_atomic(base); + + r = futex_atomic_cmpxchg_inatomic(&old_log, addr, 0, 0); + if (r < 0) + return r; + + old_log |= 1 << nr; + r = put_user(old_log, addr); + if (r < 0) + return r; + set_page_dirty_lock(page); put_page(page); return 0; @@ -1727,8 +1730,8 @@ static int log_write(void __user *log_base, 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; + u64 log = base + write_page / 32; + int bit = write_page % 32; if ((u64)(unsigned long)log != log) return -EFAULT; r = set_bit_to_user(bit, (void __user *)(unsigned long)log); -- 1.8.3.1
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