Re: [PATCH 10/22] x86/traps: Move subarch_percpu_traps_init() into traps-setup.c

[Jan Beulich <jbeulich@xxxxxxxx>]

On 13.08.2025 13:53, Andrew Cooper wrote:
> On 12/08/2025 10:52 am, Jan Beulich wrote:
>> On 11.08.2025 10:17, Andrew Cooper wrote:
>>> On 08/08/2025 9:23 pm, Andrew Cooper wrote:
>>>> ... along with the supporting functions.  Switch to Xen coding style, and 
>>>> make
>>>> static as there are no external callers.
>>>>
>>>> Rename to legacy_syscall_init() as a more accurate name.
>>>>
>>>> No functional change.
>>>>
>>>> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
>>>> ---
>>>> CC: Jan Beulich <JBeulich@xxxxxxxx>
>>>> CC: Roger Pau Monné <roger.pau@xxxxxxxxxx>
>>>> ---
>>>>  xen/arch/x86/include/asm/system.h |  2 -
>>>>  xen/arch/x86/traps-setup.c        | 97 ++++++++++++++++++++++++++++++-
>>>>  xen/arch/x86/x86_64/traps.c       | 92 -----------------------------
>>>>  3 files changed, 95 insertions(+), 96 deletions(-)
>>>>
>>>> diff --git a/xen/arch/x86/include/asm/system.h 
>>>> b/xen/arch/x86/include/asm/system.h
>>>> index 3cdc56e4ba6d..6c2800d8158d 100644
>>>> --- a/xen/arch/x86/include/asm/system.h
>>>> +++ b/xen/arch/x86/include/asm/system.h
>>>> @@ -256,6 +256,4 @@ static inline int local_irq_is_enabled(void)
>>>>  #define BROKEN_ACPI_Sx          0x0001
>>>>  #define BROKEN_INIT_AFTER_S1    0x0002
>>>>  
>>>> -void subarch_percpu_traps_init(void);
>>>> -
>>>>  #endif
>>>> diff --git a/xen/arch/x86/traps-setup.c b/xen/arch/x86/traps-setup.c
>>>> index 13b8fcf0ba51..fbae7072c292 100644
>>>> --- a/xen/arch/x86/traps-setup.c
>>>> +++ b/xen/arch/x86/traps-setup.c
>>>> @@ -2,13 +2,15 @@
>>>>  /*
>>>>   * Configuration of event handling for all CPUs.
>>>>   */
>>>> +#include <xen/domain_page.h>
>>>>  #include <xen/init.h>
>>>>  #include <xen/param.h>
>>>>  
>>>> +#include <asm/endbr.h>
>>>>  #include <asm/idt.h>
>>>>  #include <asm/msr.h>
>>>>  #include <asm/shstk.h>
>>>> -#include <asm/system.h>
>>>> +#include <asm/stubs.h>
>>>>  #include <asm/traps.h>
>>>>  
>>>>  DEFINE_PER_CPU_READ_MOSTLY(idt_entry_t *, idt);
>>>> @@ -19,6 +21,8 @@ static bool __initdata opt_ler;
>>>>  boolean_param("ler", opt_ler);
>>>>  
>>>>  void nocall entry_PF(void);
>>>> +void nocall lstar_enter(void);
>>>> +void nocall cstar_enter(void);
>>>>  
>>>>  /*
>>>>   * Sets up system tables and descriptors for IDT devliery.
>>>> @@ -138,6 +142,95 @@ static void load_system_tables(void)
>>>>      BUG_ON(stack_bottom & 15);
>>>>  }
>>>>  
>>>> +static unsigned int write_stub_trampoline(
>>>> +    unsigned char *stub, unsigned long stub_va,
>>>> +    unsigned long stack_bottom, unsigned long target_va)
>>>> +{
>>>> +    unsigned char *p = stub;
>>>> +
>>>> +    if ( cpu_has_xen_ibt )
>>>> +    {
>>>> +        place_endbr64(p);
>>>> +        p += 4;
>>>> +    }
>>>> +
>>>> +    /* Store guest %rax into %ss slot */
>>>> +    /* movabsq %rax, stack_bottom - 8 */
>>>> +    *p++ = 0x48;
>>>> +    *p++ = 0xa3;
>>>> +    *(uint64_t *)p = stack_bottom - 8;
>>>> +    p += 8;
>>>> +
>>>> +    /* Store guest %rsp in %rax */
>>>> +    /* movq %rsp, %rax */
>>>> +    *p++ = 0x48;
>>>> +    *p++ = 0x89;
>>>> +    *p++ = 0xe0;
>>>> +
>>>> +    /* Switch to Xen stack */
>>>> +    /* movabsq $stack_bottom - 8, %rsp */
>>>> +    *p++ = 0x48;
>>>> +    *p++ = 0xbc;
>>>> +    *(uint64_t *)p = stack_bottom - 8;
>>>> +    p += 8;
>>>> +
>>>> +    /* jmp target_va */
>>>> +    *p++ = 0xe9;
>>>> +    *(int32_t *)p = target_va - (stub_va + (p - stub) + 4);
>>>> +    p += 4;
>>>> +
>>>> +    /* Round up to a multiple of 16 bytes. */
>>>> +    return ROUNDUP(p - stub, 16);
>>>> +}
>>>> +
>>>> +static void legacy_syscall_init(void)
>>>> +{
>>>> +    unsigned long stack_bottom = get_stack_bottom();
>>>> +    unsigned long stub_va = this_cpu(stubs.addr);
>>>> +    unsigned char *stub_page;
>>>> +    unsigned int offset;
>>>> +
>>>> +    /* No PV guests?  No need to set up SYSCALL/SYSENTER infrastructure. 
>>>> */
>>>> +    if ( !IS_ENABLED(CONFIG_PV) )
>>>> +        return;
>>>> +
>>>> +    stub_page = map_domain_page(_mfn(this_cpu(stubs.mfn)));
>>>> +
>>>> +    /*
>>>> +     * Trampoline for SYSCALL entry from 64-bit mode.  The VT-x HVM vcpu
>>>> +     * context switch logic relies on the SYSCALL trampoline being at the
>>>> +     * start of the stubs.
>>>> +     */
>>>> +    wrmsrl(MSR_LSTAR, stub_va);
>>>> +    offset = write_stub_trampoline(stub_page + (stub_va & ~PAGE_MASK),
>>>> +                                   stub_va, stack_bottom,
>>>> +                                   (unsigned long)lstar_enter);
>>>> +    stub_va += offset;
>>>> +
>>>> +    if ( cpu_has_sep )
>>>> +    {
>>>> +        /* SYSENTER entry. */
>>>> +        wrmsrl(MSR_IA32_SYSENTER_ESP, stack_bottom);
>>>> +        wrmsrl(MSR_IA32_SYSENTER_EIP, (unsigned long)sysenter_entry);
>>>> +        wrmsr(MSR_IA32_SYSENTER_CS, __HYPERVISOR_CS, 0);
>>>> +    }
>>>> +
>>>> +    /* Trampoline for SYSCALL entry from compatibility mode. */
>>>> +    wrmsrl(MSR_CSTAR, stub_va);
>>>> +    offset += write_stub_trampoline(stub_page + (stub_va & ~PAGE_MASK),
>>>> +                                    stub_va, stack_bottom,
>>>> +                                    (unsigned long)cstar_enter);
>>>> +
>>>> +    /* Don't consume more than half of the stub space here. */
>>>> +    ASSERT(offset <= STUB_BUF_SIZE / 2);
>>>> +
>>>> +    unmap_domain_page(stub_page);
>>>> +
>>>> +    /* Common SYSCALL parameters. */
>>>> +    wrmsrl(MSR_STAR, XEN_MSR_STAR);
>>>> +    wrmsrl(MSR_SYSCALL_MASK, XEN_SYSCALL_MASK);
>>>> +}
>>> These want adjusting to use wrmsrns(), similarly to the previous patch. 
>>> Fixed locally.
>> Also the one higher in the function, I suppose.
> 
> All of them.
> 
> I'm not aware of anywhere were we want serialising behaviour, except for
> ICR which is buggly non-serialising and has workarounds.
> 
> But I'm also not sure enough of this to suggest that we make wrmsr() be
> wrmsrns() by default.

I'm pretty sure we don't want this. If nothing else then to avoid code bloat
for MSR writes which are non-serializing even in the original form.

Jan