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Date: Mon, 22 Oct 2012 11:16:15 +0200
From: Gleb Natapov <gleb@...hat.com>
To: Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>
Cc: Avi Kivity <avi@...hat.com>, Marcelo Tosatti <mtosatti@...hat.com>,
LKML <linux-kernel@...r.kernel.org>, KVM <kvm@...r.kernel.org>
Subject: Re: [PATCH] KVM: x86: fix vcpu->mmio_fragments overflow
On Fri, Oct 19, 2012 at 03:37:32PM +0800, Xiao Guangrong wrote:
> After commit b3356bf0dbb349 (KVM: emulator: optimize "rep ins" handling),
> the pieces of io data can be collected and write them to the guest memory
> or MMIO together.
>
> Unfortunately, kvm splits the mmio access into 8 bytes and store them to
> vcpu->mmio_fragments. If the guest uses "rep ins" to move large data, it
> will cause vcpu->mmio_fragments overflow
>
> The bug can be exposed by isapc (-M isapc):
>
> [23154.818733] general protection fault: 0000 [#1] SMP DEBUG_PAGEALLOC
> [ ......]
> [23154.858083] Call Trace:
> [23154.859874] [<ffffffffa04f0e17>] kvm_get_cr8+0x1d/0x28 [kvm]
> [23154.861677] [<ffffffffa04fa6d4>] kvm_arch_vcpu_ioctl_run+0xcda/0xe45 [kvm]
> [23154.863604] [<ffffffffa04f5a1a>] ? kvm_arch_vcpu_load+0x17b/0x180 [kvm]
>
>
> Actually, we can use one mmio_fragment to store a large mmio access for the
> mmio access is always continuous then split it when we pass the mmio-exit-info
> to userspace. After that, we only need two entries to store mmio info for
> the cross-mmio pages access
>
I wonder can we put the data into coalesced mmio buffer instead of
exiting for each 8 bytes? Is it worth the complexity?
> Signed-off-by: Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>
> ---
> arch/x86/kvm/x86.c | 127 +++++++++++++++++++++++++++++-----------------
> include/linux/kvm_host.h | 16 +-----
> 2 files changed, 84 insertions(+), 59 deletions(-)
>
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 8b90dd5..41ceb51 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -3779,9 +3779,6 @@ static int read_exit_mmio(struct kvm_vcpu *vcpu, gpa_t gpa,
> static int write_exit_mmio(struct kvm_vcpu *vcpu, gpa_t gpa,
> void *val, int bytes)
> {
> - struct kvm_mmio_fragment *frag = &vcpu->mmio_fragments[0];
> -
> - memcpy(vcpu->run->mmio.data, frag->data, frag->len);
> return X86EMUL_CONTINUE;
> }
>
> @@ -3799,6 +3796,64 @@ static const struct read_write_emulator_ops write_emultor = {
> .write = true,
> };
>
> +static bool get_current_mmio_info(struct kvm_vcpu *vcpu, gpa_t *gpa,
> + unsigned *len, void **data)
> +{
> + struct kvm_mmio_fragment *frag;
> + int cur = vcpu->mmio_cur_fragment;
> +
> + if (cur >= vcpu->mmio_nr_fragments)
> + return false;
> +
> + frag = &vcpu->mmio_fragments[cur];
> + if (frag->pos >= frag->len) {
> + if (++vcpu->mmio_cur_fragment >= vcpu->mmio_nr_fragments)
> + return false;
> + frag++;
> + }
> +
> + *gpa = frag->gpa + frag->pos;
> + *data = frag->data + frag->pos;
> + *len = min(8u, frag->len - frag->pos);
> + return true;
> +}
> +
> +static void complete_current_mmio(struct kvm_vcpu *vcpu)
> +{
> + struct kvm_mmio_fragment *frag;
> + gpa_t gpa;
> + unsigned len;
> + void *data;
> +
> + get_current_mmio_info(vcpu, &gpa, &len, &data);
> +
> + if (!vcpu->mmio_is_write)
> + memcpy(data, vcpu->run->mmio.data, len);
> +
> + /* Increase frag->pos to switch to the next mmio. */
> + frag = &vcpu->mmio_fragments[vcpu->mmio_cur_fragment];
> + frag->pos += len;
> +}
> +
> +static bool vcpu_fill_mmio_exit_info(struct kvm_vcpu *vcpu)
> +{
> + gpa_t gpa;
> + unsigned len;
> + void *data;
> +
> + if (!get_current_mmio_info(vcpu, &gpa, &len, &data))
> + return false;
> +
> + vcpu->run->mmio.len = len;
> + vcpu->run->mmio.is_write = vcpu->mmio_is_write;
> + vcpu->run->exit_reason = KVM_EXIT_MMIO;
> + vcpu->run->mmio.phys_addr = gpa;
> +
> + if (vcpu->mmio_is_write)
> + memcpy(vcpu->run->mmio.data, data, len);
> + return true;
> +}
> +
> static int emulator_read_write_onepage(unsigned long addr, void *val,
> unsigned int bytes,
> struct x86_exception *exception,
> @@ -3834,18 +3889,12 @@ mmio:
> bytes -= handled;
> val += handled;
>
> - while (bytes) {
> - unsigned now = min(bytes, 8U);
> -
> - frag = &vcpu->mmio_fragments[vcpu->mmio_nr_fragments++];
> - frag->gpa = gpa;
> - frag->data = val;
> - frag->len = now;
> -
> - gpa += now;
> - val += now;
> - bytes -= now;
> - }
> + WARN_ON(vcpu->mmio_nr_fragments >= KVM_MAX_MMIO_FRAGMENTS);
> + frag = &vcpu->mmio_fragments[vcpu->mmio_nr_fragments++];
> + frag->pos = 0;
> + frag->gpa = gpa;
> + frag->data = val;
> + frag->len = bytes;
> return X86EMUL_CONTINUE;
> }
>
> @@ -3855,7 +3904,6 @@ int emulator_read_write(struct x86_emulate_ctxt *ctxt, unsigned long addr,
> const struct read_write_emulator_ops *ops)
> {
> struct kvm_vcpu *vcpu = emul_to_vcpu(ctxt);
> - gpa_t gpa;
> int rc;
>
> if (ops->read_write_prepare &&
> @@ -3887,17 +3935,13 @@ int emulator_read_write(struct x86_emulate_ctxt *ctxt, unsigned long addr,
> if (!vcpu->mmio_nr_fragments)
> return rc;
>
> - gpa = vcpu->mmio_fragments[0].gpa;
> -
> vcpu->mmio_needed = 1;
> vcpu->mmio_cur_fragment = 0;
> + vcpu->mmio_is_write = ops->write;
>
> - vcpu->run->mmio.len = vcpu->mmio_fragments[0].len;
> - vcpu->run->mmio.is_write = vcpu->mmio_is_write = ops->write;
> - vcpu->run->exit_reason = KVM_EXIT_MMIO;
> - vcpu->run->mmio.phys_addr = gpa;
> -
> - return ops->read_write_exit_mmio(vcpu, gpa, val, bytes);
> + vcpu_fill_mmio_exit_info(vcpu);
> + return ops->read_write_exit_mmio(vcpu, vcpu->mmio_fragments[0].gpa,
> + val, bytes);
> }
>
> static int emulator_read_emulated(struct x86_emulate_ctxt *ctxt,
> @@ -5524,43 +5568,34 @@ static int complete_emulated_pio(struct kvm_vcpu *vcpu)
> *
> * read:
> * for each fragment
> - * write gpa, len
> - * exit
> - * copy data
> + * for each mmio piece in the fragment
> + * write gpa, len
> + * exit
> + * copy data
> * execute insn
> *
> * write:
> * for each fragment
> - * write gpa, len
> - * copy data
> - * exit
> + * for each mmio piece in the fragment
> + * write gpa, len
> + * copy data
> + * exit
> */
> static int complete_emulated_mmio(struct kvm_vcpu *vcpu)
> {
> - struct kvm_run *run = vcpu->run;
> - struct kvm_mmio_fragment *frag;
> -
> BUG_ON(!vcpu->mmio_needed);
>
> - /* Complete previous fragment */
> - frag = &vcpu->mmio_fragments[vcpu->mmio_cur_fragment++];
> - if (!vcpu->mmio_is_write)
> - memcpy(frag->data, run->mmio.data, frag->len);
> - if (vcpu->mmio_cur_fragment == vcpu->mmio_nr_fragments) {
> + complete_current_mmio(vcpu);
> +
> + /* Initiate next fragment */
> + if (!vcpu_fill_mmio_exit_info(vcpu)) {
> vcpu->mmio_needed = 0;
> if (vcpu->mmio_is_write)
> return 1;
> vcpu->mmio_read_completed = 1;
> return complete_emulated_io(vcpu);
> }
> - /* Initiate next fragment */
> - ++frag;
> - run->exit_reason = KVM_EXIT_MMIO;
> - run->mmio.phys_addr = frag->gpa;
> - if (vcpu->mmio_is_write)
> - memcpy(run->mmio.data, frag->data, frag->len);
> - run->mmio.len = frag->len;
> - run->mmio.is_write = vcpu->mmio_is_write;
> +
> vcpu->arch.complete_userspace_io = complete_emulated_mmio;
> return 0;
> }
> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> index e235068..f6c3c2f 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -42,19 +42,8 @@
> */
> #define KVM_MEMSLOT_INVALID (1UL << 16)
>
> -/*
> - * If we support unaligned MMIO, at most one fragment will be split into two:
> - */
> -#ifdef KVM_UNALIGNED_MMIO
> -# define KVM_EXTRA_MMIO_FRAGMENTS 1
> -#else
> -# define KVM_EXTRA_MMIO_FRAGMENTS 0
> -#endif
> -
> -#define KVM_USER_MMIO_SIZE 8
> -
> -#define KVM_MAX_MMIO_FRAGMENTS \
> - (KVM_MMIO_SIZE / KVM_USER_MMIO_SIZE + KVM_EXTRA_MMIO_FRAGMENTS)
> +/* Two fragments for cross MMIO pages. */
> +#define KVM_MAX_MMIO_FRAGMENTS 2
>
> /*
> * For the normal pfn, the highest 12 bits should be zero,
> @@ -203,6 +192,7 @@ struct kvm_mmio_fragment {
> gpa_t gpa;
> void *data;
> unsigned len;
> + unsigned pos;
> };
>
> struct kvm_vcpu {
> --
> 1.7.7.6
--
Gleb.
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