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

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

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.

> Acked-by: Jan Beulich <jbeulich@xxxxxxxx>

Thanks.

~Andrew