Re: [PATCH v2 3/7] KVM: SVM: refactor msr permission bitmap allocation

[Jim Mattson]

On Thu, Aug 20, 2020 at 6:34 AM Maxim Levitsky <mlevitsk@xxxxxxxxxx> wrote:
>
> Replace svm_vcpu_init_msrpm with svm_vcpu_alloc_msrpm, that also allocates
> the msr bitmap and add svm_vcpu_free_msrpm to free it.
>
> This will be used later to move the nested msr permission bitmap allocation
> to nested.c
>
> No functional change intended.
>
> Signed-off-by: Maxim Levitsky <mlevitsk@xxxxxxxxxx>
> ---
>  arch/x86/kvm/svm/svm.c | 45 +++++++++++++++++++++---------------------
>  1 file changed, 23 insertions(+), 22 deletions(-)
>
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index d33013b9b4d7..7bb094bf6494 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -609,18 +609,29 @@ static void set_msr_interception(u32 *msrpm, unsigned msr,
>         msrpm[offset] = tmp;
>  }
>
> -static void svm_vcpu_init_msrpm(u32 *msrpm)
> +static u32 *svm_vcpu_alloc_msrpm(void)

I prefer the original name, since this function does more than allocation.

>  {
>         int i;
> +       u32 *msrpm;
> +       struct page *pages = alloc_pages(GFP_KERNEL_ACCOUNT, MSRPM_ALLOC_ORDER);
> +
> +       if (!pages)
> +               return NULL;
>
> +       msrpm = page_address(pages);
>         memset(msrpm, 0xff, PAGE_SIZE * (1 << MSRPM_ALLOC_ORDER));
>
>         for (i = 0; direct_access_msrs[i].index != MSR_INVALID; i++) {
>                 if (!direct_access_msrs[i].always)
>                         continue;
> -
>                 set_msr_interception(msrpm, direct_access_msrs[i].index, 1, 1);
>         }
> +       return msrpm;
> +}
> +
> +static void svm_vcpu_free_msrpm(u32 *msrpm)
> +{
> +       __free_pages(virt_to_page(msrpm), MSRPM_ALLOC_ORDER);
>  }
>
>  static void add_msr_offset(u32 offset)
> @@ -1172,9 +1183,7 @@ static int svm_create_vcpu(struct kvm_vcpu *vcpu)
>  {
>         struct vcpu_svm *svm;
>         struct page *vmcb_page;
> -       struct page *msrpm_pages;
>         struct page *hsave_page;
> -       struct page *nested_msrpm_pages;
>         int err;
>
>         BUILD_BUG_ON(offsetof(struct vcpu_svm, vcpu) != 0);
> @@ -1185,21 +1194,13 @@ static int svm_create_vcpu(struct kvm_vcpu *vcpu)
>         if (!vmcb_page)
>                 goto out;
>
> -       msrpm_pages = alloc_pages(GFP_KERNEL_ACCOUNT, MSRPM_ALLOC_ORDER);
> -       if (!msrpm_pages)
> -               goto free_page1;
> -
> -       nested_msrpm_pages = alloc_pages(GFP_KERNEL_ACCOUNT, MSRPM_ALLOC_ORDER);
> -       if (!nested_msrpm_pages)
> -               goto free_page2;
> -

Reordering the allocations does seem like a functional change to me,
albeit one that should (hopefully) be benign. For example, if the
MSRPM_ALLOC_ORDER allocations fail, in the new version of the code,
the hsave_page will be cleared, but in the old version of the code, no
page would be cleared.

>         hsave_page = alloc_page(GFP_KERNEL_ACCOUNT);

Speaking of clearing pages, why not add __GFP_ZERO to the flags above
and skip the clear_page() call below?

>         if (!hsave_page)
> -               goto free_page3;
> +               goto free_page1;
>
>         err = avic_init_vcpu(svm);
>         if (err)
> -               goto free_page4;
> +               goto free_page2;
>
>         /* We initialize this flag to true to make sure that the is_running
>          * bit would be set the first time the vcpu is loaded.
> @@ -1210,11 +1211,13 @@ static int svm_create_vcpu(struct kvm_vcpu *vcpu)
>         svm->nested.hsave = page_address(hsave_page);
>         clear_page(svm->nested.hsave);
>
> -       svm->msrpm = page_address(msrpm_pages);
> -       svm_vcpu_init_msrpm(svm->msrpm);
> +       svm->msrpm = svm_vcpu_alloc_msrpm();
> +       if (!svm->msrpm)
> +               goto free_page2;
>
> -       svm->nested.msrpm = page_address(nested_msrpm_pages);
> -       svm_vcpu_init_msrpm(svm->nested.msrpm);
> +       svm->nested.msrpm = svm_vcpu_alloc_msrpm();
> +       if (!svm->nested.msrpm)
> +               goto free_page3;
>
>         svm->vmcb = page_address(vmcb_page);
>         clear_page(svm->vmcb);
> @@ -1227,12 +1230,10 @@ static int svm_create_vcpu(struct kvm_vcpu *vcpu)
>
>         return 0;
>
> -free_page4:
> -       __free_page(hsave_page);
>  free_page3:
> -       __free_pages(nested_msrpm_pages, MSRPM_ALLOC_ORDER);
> +       svm_vcpu_free_msrpm(svm->msrpm);
>  free_page2:
> -       __free_pages(msrpm_pages, MSRPM_ALLOC_ORDER);
> +       __free_page(hsave_page);
>  free_page1:
>         __free_page(vmcb_page);
>  out:

While you're here, could you improve these labels? Coding-style.rst says:

Choose label names which say what the goto does or why the goto exists.  An
example of a good name could be ``out_free_buffer:`` if the goto frees
``buffer``.
Avoid using GW-BASIC names like ``err1:`` and ``err2:``, as you would have to
renumber them if you ever add or remove exit paths, and they make correctness
difficult to verify anyway.