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Message-ID: <f95cd8c6-af5c-4d8f-99a8-16d0ec56d9a4@linux.intel.com>
Date: Mon, 4 Nov 2024 16:49:52 +0800
From: Binbin Wu <binbin.wu@...ux.intel.com>
To: Sean Christopherson <seanjc@...gle.com>, Kai Huang <kai.huang@...el.com>
Cc: Rick P Edgecombe <rick.p.edgecombe@...el.com>,
Xiaoyao Li <xiaoyao.li@...el.com>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
"yuan.yao@...ux.intel.com" <yuan.yao@...ux.intel.com>,
"kvm@...r.kernel.org" <kvm@...r.kernel.org>,
Isaku Yamahata <isaku.yamahata@...el.com>,
"pbonzini@...hat.com" <pbonzini@...hat.com>
Subject: Re: [PATCH v3 1/2] KVM: x86: Check hypercall's exit to userspace
generically
On 11/2/2024 12:39 AM, Sean Christopherson wrote:
> On Fri, Nov 01, 2024, Kai Huang wrote:
>> On Thu, 2024-10-31 at 07:54 -0700, Sean Christopherson wrote:
>>> On Thu, Oct 31, 2024, Kai Huang wrote:
>>> - ret = __kvm_emulate_hypercall(vcpu, nr, a0, a1, a2, a3, op_64_bit, cpl);
>>> - if (nr == KVM_HC_MAP_GPA_RANGE && !ret)
>>> - /* MAP_GPA tosses the request to the user space. */
>>> - return 0;
>>> + r = __kvm_emulate_hypercall(vcpu, nr, a0, a1, a2, a3, op_64_bit, cpl, &ret);
>>> + if (r <= r)
>>> + return r;
>> ... should be:
>>
>> if (r <= 0)
>> return r;
>>
>> ?
>>
>> Another option might be we move "set hypercall return value" code inside
>> __kvm_emulate_hypercall(). So IIUC the reason to split
>> __kvm_emulate_hypercall() out is for TDX, and while non-TDX uses RAX to carry
>> the hypercall return value, TDX uses R10.
>>
>> We can additionally pass a "kvm_hypercall_set_ret_func" function pointer to
>> __kvm_emulate_hypercall(), and invoke it inside. Then we can change
>> __kvm_emulate_hypercall() to return:
>> < 0 error,
>> ==0 return to userspace,
>> > 0 go back to guest.
> Hmm, and the caller can still handle kvm_skip_emulated_instruction(), because the
> return value is KVM's normal pattern.
>
> I like it!
>
> But, there's no need to pass a function pointer, KVM can write (and read) arbitrary
> GPRs, it's just avoided in most cases so that the sanity checks and available/dirty
> updates are elided. For this code though, it's easy enough to keep kvm_rxx_read()
> for getting values, and eating the overhead of a single GPR write is a perfectly
> fine tradeoff for eliminating the return multiplexing.
>
> Lightly tested. Assuming this works for TDX and passes testing, I'll post a
> mini-series next week.
>
> --
> From: Sean Christopherson <seanjc@...gle.com>
> Date: Fri, 1 Nov 2024 09:04:00 -0700
> Subject: [PATCH] KVM: x86: Refactor __kvm_emulate_hypercall() to accept reg
> names, not values
>
> Rework __kvm_emulate_hypercall() to take the names of input and output
> (guest return value) registers, as opposed to taking the input values and
> returning the output value. As part of the refactor, change the actual
> return value from __kvm_emulate_hypercall() to be KVM's de facto standard
> of '0' == exit to userspace, '1' == resume guest, and -errno == failure.
>
> Using the return value for KVM's control flow eliminates the multiplexed
> return value, where '0' for KVM_HC_MAP_GPA_RANGE (and only that hypercall)
> means "exit to userspace".
>
> Use the direct GPR accessors to read values to avoid the pointless marking
> of the registers as available, but use kvm_register_write_raw() for the
> guest return value so that the innermost helper doesn't need to multiplex
> its return value. Using the generic kvm_register_write_raw() adds very
> minimal overhead, so as a one-off in a relatively slow path it's well
> worth the code simplification.
>
> Suggested-by: Kai Huang <kai.huang@...el.com>
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> ---
> arch/x86/include/asm/kvm_host.h | 15 +++++++++----
> arch/x86/kvm/x86.c | 40 +++++++++++++--------------------
> 2 files changed, 27 insertions(+), 28 deletions(-)
>
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index 6d9f763a7bb9..9e66fde1c4e4 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -2179,10 +2179,17 @@ static inline void kvm_clear_apicv_inhibit(struct kvm *kvm,
> kvm_set_or_clear_apicv_inhibit(kvm, reason, false);
> }
>
> -unsigned long __kvm_emulate_hypercall(struct kvm_vcpu *vcpu, unsigned long nr,
> - unsigned long a0, unsigned long a1,
> - unsigned long a2, unsigned long a3,
> - int op_64_bit, int cpl);
> +int ____kvm_emulate_hypercall(struct kvm_vcpu *vcpu, unsigned long nr,
> + unsigned long a0, unsigned long a1,
> + unsigned long a2, unsigned long a3,
> + int op_64_bit, int cpl, int ret_reg);
> +
> +#define __kvm_emulate_hypercall(_vcpu, nr, a0, a1, a2, a3, op_64_bit, cpl, ret) \
> + ____kvm_emulate_hypercall(vcpu, \
> + kvm_##nr##_read(vcpu), kvm_##a0##_read(vcpu), \
> + kvm_##a1##_read(vcpu), kvm_##a2##_read(vcpu), \
> + kvm_##a3##_read(vcpu), op_64_bit, cpl, VCPU_REGS_##ret)
> +
> int kvm_emulate_hypercall(struct kvm_vcpu *vcpu);
>
> int kvm_mmu_page_fault(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa, u64 error_code,
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index e09daa3b157c..425a301911a6 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -9998,10 +9998,10 @@ static int complete_hypercall_exit(struct kvm_vcpu *vcpu)
> return kvm_skip_emulated_instruction(vcpu);
> }
>
> -unsigned long __kvm_emulate_hypercall(struct kvm_vcpu *vcpu, unsigned long nr,
> - unsigned long a0, unsigned long a1,
> - unsigned long a2, unsigned long a3,
> - int op_64_bit, int cpl)
> +int ____kvm_emulate_hypercall(struct kvm_vcpu *vcpu, unsigned long nr,
> + unsigned long a0, unsigned long a1,
> + unsigned long a2, unsigned long a3,
> + int op_64_bit, int cpl, int ret_reg)
> {
> unsigned long ret;
>
> @@ -10086,15 +10086,18 @@ unsigned long __kvm_emulate_hypercall(struct kvm_vcpu *vcpu, unsigned long nr,
>
> out:
> ++vcpu->stat.hypercalls;
> - return ret;
> +
> + if (!op_64_bit)
> + ret = (u32)ret;
> +
> + kvm_register_write_raw(vcpu, ret_reg, ret);
> + return 1;
> }
> -EXPORT_SYMBOL_GPL(__kvm_emulate_hypercall);
> +EXPORT_SYMBOL_GPL(____kvm_emulate_hypercall);
>
> int kvm_emulate_hypercall(struct kvm_vcpu *vcpu)
> {
> - unsigned long nr, a0, a1, a2, a3, ret;
> - int op_64_bit;
> - int cpl;
> + int r;
>
> if (kvm_xen_hypercall_enabled(vcpu->kvm))
> return kvm_xen_hypercall(vcpu);
> @@ -10102,23 +10105,12 @@ int kvm_emulate_hypercall(struct kvm_vcpu *vcpu)
> if (kvm_hv_hypercall_enabled(vcpu))
> return kvm_hv_hypercall(vcpu);
>
> - nr = kvm_rax_read(vcpu);
> - a0 = kvm_rbx_read(vcpu);
> - a1 = kvm_rcx_read(vcpu);
> - a2 = kvm_rdx_read(vcpu);
> - a3 = kvm_rsi_read(vcpu);
> - op_64_bit = is_64_bit_hypercall(vcpu);
> - cpl = kvm_x86_call(get_cpl)(vcpu);
> -
> - ret = __kvm_emulate_hypercall(vcpu, nr, a0, a1, a2, a3, op_64_bit, cpl);
> - if (nr == KVM_HC_MAP_GPA_RANGE && !ret)
> - /* MAP_GPA tosses the request to the user space. */
> + r = __kvm_emulate_hypercall(vcpu, rax, rbx, rcx, rdx, rsi,
> + is_64_bit_hypercall(vcpu),
> + kvm_x86_call(get_cpl)(vcpu), RAX);
Now, the register for return code of the hypercall can be specified.
But in ____kvm_emulate_hypercall(), the complete_userspace_io callback
is hardcoded to complete_hypercall_exit(), which always set return code
to RAX.
We can allow the caller to pass in the cui callback, or assign different
version according to the input 'ret_reg'. So that different callers can use
different cui callbacks. E.g., TDX needs to set return code to R10 in cui
callback.
How about:
diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index dba78f22ab27..0fba98685f42 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -2226,13 +2226,15 @@ static inline void kvm_clear_apicv_inhibit(struct kvm *kvm,
int ____kvm_emulate_hypercall(struct kvm_vcpu *vcpu, unsigned long nr,
unsigned long a0, unsigned long a1,
unsigned long a2, unsigned long a3,
- int op_64_bit, int cpl, int ret_reg);
+ int op_64_bit, int cpl, int ret_reg,
+ int (*cui)(struct kvm_vcpu *vcpu));
-#define __kvm_emulate_hypercall(_vcpu, nr, a0, a1, a2, a3, op_64_bit, cpl, ret) \
+#define __kvm_emulate_hypercall(_vcpu, nr, a0, a1, a2, a3, op_64_bit, cpl, ret, cui) \
____kvm_emulate_hypercall(vcpu, \
kvm_##nr##_read(vcpu), kvm_##a0##_read(vcpu), \
kvm_##a1##_read(vcpu), kvm_##a2##_read(vcpu), \
- kvm_##a3##_read(vcpu), op_64_bit, cpl, VCPU_REGS_##ret)
+ kvm_##a3##_read(vcpu), op_64_bit, cpl, VCPU_REGS_##ret, \
+ cui)
int kvm_emulate_hypercall(struct kvm_vcpu *vcpu);
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 6e0a518aec4a..b68690c4a4c0 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -10019,7 +10019,8 @@ static int complete_hypercall_exit(struct kvm_vcpu *vcpu)
int ____kvm_emulate_hypercall(struct kvm_vcpu *vcpu, unsigned long nr,
unsigned long a0, unsigned long a1,
unsigned long a2, unsigned long a3,
- int op_64_bit, int cpl, int ret_reg)
+ int op_64_bit, int cpl, int ret_reg,
+ int (*cui)(struct kvm_vcpu *vcpu))
{
unsigned long ret;
@@ -10093,7 +10094,7 @@ int ____kvm_emulate_hypercall(struct kvm_vcpu *vcpu, unsigned long nr,
vcpu->run->hypercall.flags |= KVM_EXIT_HYPERCALL_LONG_MODE;
WARN_ON_ONCE(vcpu->run->hypercall.flags & KVM_EXIT_HYPERCALL_MBZ);
- vcpu->arch.complete_userspace_io = complete_hypercall_exit;
+ vcpu->arch.complete_userspace_io = cui;
/* stat is incremented on completion. */
return 0;
}
@@ -10125,7 +10126,7 @@ int kvm_emulate_hypercall(struct kvm_vcpu *vcpu)
r = __kvm_emulate_hypercall(vcpu, rax, rbx, rcx, rdx, rsi,
is_64_bit_hypercall(vcpu),
- kvm_x86_call(get_cpl)(vcpu), RAX);
+ kvm_x86_call(get_cpl)(vcpu), RAX, complete_hypercall_exit);
if (r <= 0)
return 0;
> + if (r <= 0)
> return 0;
>
> - if (!op_64_bit)
> - ret = (u32)ret;
> - kvm_rax_write(vcpu, ret);
> -
> return kvm_skip_emulated_instruction(vcpu);
> }
> EXPORT_SYMBOL_GPL(kvm_emulate_hypercall);
>
> base-commit: 911785b796e325dec83b32050f294e278a306211
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