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    Subject[PATCH 4/4] kvm: reduce stack usage in kvm_pv_mmu_op()
    Date
    This time, we're in a hot path.  We can't use kmalloc() because
    it might impact performance. So, we just stick the buffer that
    we need into the kvm_vcpu_arch structure. This is used very
    often, so it is not really a waste.

    We also have to move the buffer structure's definition to the
    arch-specific x86 kvm header.

    Signed-off-by: Dave Hansen <dave@linux.vnet.ibm.com>
    ---
    arch/x86/kvm/mmu.c | 23 ++++++++---------------
    include/asm-x86/kvm_host.h | 10 ++++++++++
    2 files changed, 18 insertions(+), 15 deletions(-)

    diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
    index 0bfe2bd..c2ce32c 100644
    --- a/arch/x86/kvm/mmu.c
    +++ b/arch/x86/kvm/mmu.c
    @@ -135,13 +135,6 @@ module_param(dbg, bool, 0644);
    #define ACC_USER_MASK PT_USER_MASK
    #define ACC_ALL (ACC_EXEC_MASK | ACC_WRITE_MASK | ACC_USER_MASK)

    -struct kvm_pv_mmu_op_buffer {
    - void *ptr;
    - unsigned len;
    - unsigned processed;
    - char buf[512] __aligned(sizeof(long));
    -};
    -
    struct kvm_rmap_desc {
    u64 *shadow_ptes[RMAP_EXT];
    struct kvm_rmap_desc *more;
    @@ -2287,18 +2280,18 @@ int kvm_pv_mmu_op(struct kvm_vcpu *vcpu, unsigned long bytes,
    gpa_t addr, unsigned long *ret)
    {
    int r;
    - struct kvm_pv_mmu_op_buffer buffer;
    + struct kvm_pv_mmu_op_buffer *buffer = &vcpu->arch.mmu_op_buffer;

    - buffer.ptr = buffer.buf;
    - buffer.len = min_t(unsigned long, bytes, sizeof buffer.buf);
    - buffer.processed = 0;
    + buffer->ptr = buffer->buf;
    + buffer->len = min_t(unsigned long, bytes, sizeof buffer->buf);
    + buffer->processed = 0;

    - r = kvm_read_guest(vcpu->kvm, addr, buffer.buf, buffer.len);
    + r = kvm_read_guest(vcpu->kvm, addr, buffer->buf, buffer->len);
    if (r)
    goto out;

    - while (buffer.len) {
    - r = kvm_pv_mmu_op_one(vcpu, &buffer);
    + while (buffer->len) {
    + r = kvm_pv_mmu_op_one(vcpu, buffer);
    if (r < 0)
    goto out;
    if (r == 0)
    @@ -2307,7 +2300,7 @@ int kvm_pv_mmu_op(struct kvm_vcpu *vcpu, unsigned long bytes,

    r = 1;
    out:
    - *ret = buffer.processed;
    + *ret = buffer->processed;
    return r;
    }

    diff --git a/include/asm-x86/kvm_host.h b/include/asm-x86/kvm_host.h
    index 0f3c531..ab0b351 100644
    --- a/include/asm-x86/kvm_host.h
    +++ b/include/asm-x86/kvm_host.h
    @@ -195,6 +195,13 @@ struct kvm_mmu_page {
    };
    };

    +struct kvm_pv_mmu_op_buffer {
    + void *ptr;
    + unsigned len;
    + unsigned processed;
    + char buf[512] __aligned(sizeof(long));
    +};
    +
    /*
    * x86 supports 3 paging modes (4-level 64-bit, 3-level 64-bit, and 2-level
    * 32-bit). The kvm_mmu structure abstracts the details of the current mmu
    @@ -237,6 +244,9 @@ struct kvm_vcpu_arch {
    bool tpr_access_reporting;

    struct kvm_mmu mmu;
    + /* only needed in kvm_pv_mmu_op() path, but it's hot so
    + * put it here to avoid allocation */
    + struct kvm_pv_mmu_op_buffer mmu_op_buffer;

    struct kvm_mmu_memory_cache mmu_pte_chain_cache;
    struct kvm_mmu_memory_cache mmu_rmap_desc_cache;
    --
    1.5.4.3


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