Re: [PATCH v2 03/39] xen/riscv: set the guest's XLEN explicitly in hstatus.VSXL

[Oleksii Kurochko <oleksii.kurochko@gmail.com>]

On 9/1/26 5:20 PM, Jan Beulich wrote:
> On 27.08.2026 17:20, Oleksii Kurochko wrote:
>> --- a/xen/arch/riscv/domain.c
>> +++ b/xen/arch/riscv/domain.c
>> @@ -88,7 +88,13 @@ static void vcpu_csr_init(struct vcpu *v)
>>   {
>>       v->arch.hedeleg = HEDELEG_DEFAULT & csr_masks.hedeleg;
>>   
>> -    vcpu_guest_cpu_user_regs(v)->hstatus = HSTATUS_SPV | HSTATUS_SPVP;
>> +    /*
>> +     * Xen supports 64-bit guests only, so set the guest's XLEN explicitly
>> +     * rather than leaving it to the WARL behaviour of hstatus.VSXL, which the
>> +     * decoding of a trapped instruction depends on.
>> +     */
>> +    vcpu_guest_cpu_user_regs(v)->hstatus =
>> +        HSTATUS_SPV | HSTATUS_SPVP | MASK_INSR(HSTATUS_VSXL_64, HSTATUS_VSXL);
> 
> The comment is correct right now, but the situation better would change
> at some point. Can't you arrange for things to be correct here also for
> a future where 32- and 128-bit guests would also be supported?

I am not sure about 128-bit guests as H extension is dependent on RV32 
or RV64 but probably it will be changed:
```
The hypervisor extension depends on an "I" base integer ISA with 32 x 
registers (RV32I or RV64I), not RV32E or RV64E, which have only 16 x 
registers.
```

I will introduce the following (also it will be needed also to check if 
we could VSXL set at all as implmentation can make that field read-only 
and do VSXLLEN=HSXLEN):

/*
  * Return the hstatus.VSXL value encoding the guest's XLEN. The switch()
  * deliberately has no default case, so that adding a new domain_type (a
  * 128-bit one, in particular) fails to build until this mapping is 
updated.
  */
static unsigned int domain_vsxl(const struct domain *d)
{
     switch ( d->type )
     {
     case DOMAIN_32BIT:
         return HSTATUS_VSXL_32;

     case DOMAIN_64BIT:
         return HSTATUS_VSXL_64;
     }

     ASSERT_UNREACHABLE();

     return HSTATUS_VSXL_64;
}

It will also affect then common code as vcpu_csr_init() could be then 
called before domain type is set:

+++ b/xen/common/device-tree/dom0less-build.c
@@ -812,17 +812,18 @@ static int __init construct_domU(struct 
kernel_info *kinfo,
      else if ( rc == 0 && !strcmp(dom0less_enhanced, "no-xenstore") )
          kinfo->dom0less_feature = DOM0LESS_ENHANCED_NO_XS;

-    if ( vcpu_create(d, 0) == NULL )
-        return -ENOMEM;
-
      d->max_pages = ((paddr_t)mem * SZ_1K) >> PAGE_SHIFT;

      rc = kernel_probe(kinfo, node);
      if ( rc < 0 )
          return rc;

+    /* The domain type needs to be known before the first vCPU is 
created. */
      set_domain_type(d, kinfo);

+    if ( vcpu_create(d, 0) == NULL )
+        return -ENOMEM;


> 
>> --- a/xen/arch/riscv/include/asm/riscv_encoding.h
>> +++ b/xen/arch/riscv/include/asm/riscv_encoding.h
>> @@ -68,6 +68,8 @@
>>   #if __riscv_xlen == 64
>>   #define HSTATUS_VSXL			_UL(0x300000000)
>>   #define HSTATUS_VSXL_SHIFT		32
>> +#define HSTATUS_VSXL_64			_UL(2)
>> +#define HSTATUS_VSXL_32			_UL(1)
>>   #endif
> 
> While adding the two #define-s, would you mind considering to remove the
> unused (and supposed to remain so) HSTATUS_VSXL_SHIFT?

Sure, I will drop that.

Thanks.

~ Oleksii