Re: [PATCH v5 3/5] hyperv: Enable the hypercall output page for the VTL mode

[Roman Kisel]

On 1/8/2025 11:17 AM, Stanislav Kinsburskii wrote:

On Tue, Jan 07, 2025 at 03:11:15PM -0800, Roman Kisel wrote:

[...]

Avoiding using the output hypercall page leads to something like[1]
and it looks quite complicated although that's the bare bones, lots
of notes.

How is this related to the original discussion?

I was looking for ways to eliminate what I perceived as the source of
friction in the discussion -- allocating the hypercall output page.

My concern was about the piece allocating of the output page guarded by
the VTL config option.>> Thanks,
Stas

[1]

/*
  * Fast extended hypercall with 20 bytes of input and 16 bytes of
  * output for getting a VP register.
  *
  * NOTES:
  *  1. The function is __init only atm, so the XMM context isn't
  *     used by the user mode.
  *  2. X86_64 only.
  *  3. Fast extended hypercalls may use XMM0..XMM6, and XMM is
  *     architerctural on X86_64 yet the support should be enabled
  *     in the CR's. Here, need RDX, R8 and XMM0 for input and RDX,
  *     R8 for output
  *  4. No provisions for TDX and SEV-SNP for the sake of simplicity
  *     (the hypervisor cannot see the guest registers in the
  *     confidential VM), would need to fallback.
  *  5. The robust implementation would need to check if fast extended
  *     hypercalls are available by checking the synthehtic CPUID leaves.
  *     A separate leaf indicates fast output support.
  *     It _almost_ certainly has to be, unless somehow disabled, hard
  *     to see why that would be needed.
  */
struct hv_u128 {
        u64 low_part;
        u64 high_part;
} __packed;

static __init u64 hv_vp_get_register_xfast(u32 reg_name,
                struct hv_u128 *value)
{
        u64 control = HV_HYPERCALL_REP_COMP_1 | HVCALL_GET_VP_REGISTERS |
                                        HV_HYPERCALL_FAST_BIT;
        unsigned long flags;
        u64 hv_status;

        union {
                struct hv_get_vp_registers_input input;
                struct {
                        u64 lo;
                        u64 hi;
                } __packed as_u128;
        } hv_input;
        register u64 rdx asm("rdx");
        register u64 r8 asm("r8");
        register u64 r12 asm("r12");

        local_irq_save(flags);

        hv_input.as_u128.lo = hv_input.as_u128.hi = 0;
        hv_input.input.header.partitionid = HV_PARTITION_ID_SELF;
        hv_input.input.header.vpindex = HV_VP_INDEX_SELF;
        hv_input.input.header.inputvtl = 0;

        rdx = hv_input.as_u128.lo;
        r8 = hv_input.as_u128.hi;
        r12 = reg_name;

        __asm__ __volatile__(
                        "subq              $16, %%rsp\n"
                        "movups            %%xmm0, 16(%%rsp)\n"
                        "movd              %%r12, %%xmm0\n"
                        CALL_NOSPEC
                        "movups            16(%%rsp), %%xmm0\n"
                        "addq              $16, %%rsp\n"
                        : "=a" (hv_status), ASM_CALL_CONSTRAINT,
                        "+c" (control), "+r" (rdx), "+r" (r8)
                        : THUNK_TARGET(hv_hypercall_pg), "r"(r12)
                        : "cc", "memory", "r9", "r10", "r11");

        if (hv_result_success(hv_status)) {
                value->low_part = rdx;
                value->high_part = r8;
        }

        local_irq_restore(flags);
        return hv_status;
}

#if IS_ENABLED(CONFIG_HYPERV_VTL_MODE)
u8 __init get_vtl(void)
{
        struct hv_u128 reg_value;
        u64 ret = hv_vp_get_register_xfast(HV_REGISTER_VSM_VP_STATUS, 
&reg_value);

        if (hv_result_success(ret)) {
                ret = reg_value.low_part & HV_VTL_MASK;
        } else {
                pr_err("Failed to get VTL(error: %lld) exiting...\n", ret);
                BUG();
        }

        return ret;
}
#endif

Thanks,
Stas

--
Thank you,
Roman

--
Thank you,
Roman